Chapter 12: Sexual Reproduction and Meiosis

225

12Sexual Reproduction

and Meiosis

Concept Outline

12.1 Meiosis produces haploid cells from diploid cells.

Discovery of Reduction Division. Sexual reproductiondoes not increase chromosome number because gameteproduction by meiosis involves a decrease in chromosomenumber. Individuals produced from sexual reproductioninherit chromosomes from two parents.

12.2 Meiosis has three unique features.

Unique Features of Meiosis. Three unique features ofmeiosis are synapsis, homologous recombination, andreduction division.

12.3 The sequence of events during meiosis involvestwo nuclear divisions.

Prophase I. Homologous chromosomes pair intimately,and undergo crossing over that locks them together.Metaphase I. Spindle microtubules align thechromosomes in the central plane of the cell.Completing Meiosis. The second meiotic division is likea mitotic division, but has a very different outcome.

12.4 The evolutionary origin of sex is a puzzle.

Why Sex? Sex may have evolved as a mechanism to repairDNA, or perhaps as a means for contagious elements tospread. Sexual reproduction increases genetic variability byshuffling combinations of genes. Most animals and plants reproduce sexually. Gametes

of opposite sex unite to form a cell that, dividing re-peatedly by mitosis, eventually gives rise to an adult bodywith some 100 trillion cells. The gametes that give rise tothe initial cell are the products of a special form of cell divi-sion called meiosis (figure 12.1), the subject of this chapter.Far more intricate than mitosis, the details of meiosis arenot as well understood. The basic process, however, isclear. Also clear are the profound consequences of sexualreproduction: it plays a key role in generating the tremen-dous genetic diversity that is the raw material of evolution.

FIGURE 12.1Plant cells undergoing meiosis (600×). This preparation ofpollen cells of a spiderwort, Tradescantia, was made by freezingthe cells and then fracturing them. It shows several stages ofmeiosis.

number of chromosomes in each cell would become impos-sibly large. For example, in just 10 generations, the 46chromosomes present in human cells would increase toover 47,000 (46 × 210).

The number of chromosomes does not explode in thisway because of a special reduction division that occursduring gamete formation, producing cells with half thenormal number of chromosomes. The subsequent fusionof two of these cells ensures a consistent chromosomenumber from one generation to the next. This reductiondivision process, known as meiosis, is the subject of thischapter.

The Sexual Life Cycle

Meiosis and fertilization together constitute a cycle of re-production. Two sets of chromosomes are present in thesomatic cells of adult individuals, making them diploidcells (Greek diploos, “double” + eidos, “form”), but only oneset is present in the gametes, which are thus haploid(Greek haploos, “single” + ploion, “vessel”). Reproductionthat involves this alternation of meiosis and fertilization iscalled sexual reproduction. Its outstanding characteristicis that offspring inherit chromosomes from two parents(figure 12.2). You, for example, inherited 23 chromosomesfrom your mother, contributed by the egg fertilized at yourconception, and 23 from your father, contributed by thesperm that fertilized that egg.

226 Part IV Reproduction and Heredity

Discovery of Reduction DivisionOnly a few years after Walther Fleming’s discovery ofchromosomes in 1882, Belgian cytologist Pierre-Joseph vanBeneden was surprised to find different numbers of chro-mosomes in different types of cells in the roundworm As-caris. Specifically, he observed that the gametes (eggs andsperm) each contained two chromosomes, while the so-matic (nonreproductive) cells of embryos and mature indi-viduals each contained four.

Fertilization

From his observations, van Beneden proposed in 1887 thatan egg and a sperm, each containing half the complementof chromosomes found in other cells, fuse to produce a sin-gle cell called a zygote. The zygote, like all of the somaticcells ultimately derived from it, contains two copies of eachchromosome. The fusion of gametes to form a new cell iscalled fertilization, or syngamy.

Reduction Division

It was clear even to early investigators that gamete forma-tion must involve some mechanism that reduces the num-ber of chromosomes to half the number found in othercells. If it did not, the chromosome number would doublewith each fertilization, and after only a few generations, the


Haploid eggDiploid zygote

Haploid sperm

FIGURE 12.2Diploid cells carry chromosomes from twoparents. A diploid cell contains two versions ofeach chromosome, one contributed by the haploidegg of the mother, the other by the haploid spermof the father.

Somatic Tissues. The life cycles of all sexually reproduc-ing organisms follow the same basic pattern of alternationbetween the diploid and haploid chromosome numbers(figures 12.3 and 12.4). After fertilization, the resulting zy-gote begins to divide by mitosis. This single diploid celleventually gives rise to all of the cells in the adult. Thesecells are called somatic cells, from the Latin word for“body.” Except when rare accidents occur, or in specialvariation-creating situations such as occur in the immunesystem, every one of the adult’s somatic cells is geneticallyidentical to the zygote.

In unicellular eukaryotic organisms, including most pro-tists, individual cells function as gametes, fusing with othergamete cells. The zygote may undergo mitosis, or it maydivide immediately by meiosis to give rise to haploid indi-viduals. In plants, the haploid cells that meiosis producesdivide by mitosis, forming a multicellular haploid phase.Certain cells of this haploid phase eventually differentiateinto eggs or sperm.

Germ-Line Tissues. In animals, the cells that will eventu-ally undergo meiosis to produce gametes are set aside fromsomatic cells early in the course of development. These cellsare often referred to as germ-line cells. Both the somaticcells and the gamete-producing germ-line cells are diploid,but while somatic cells undergo mitosis to form geneticallyidentical, diploid daughter cells, gamete-producing germ-line cells undergo meiosis, producing haploid gametes.

Meiosis is a process of cell division in which the numberof chromosomes in certain cells is halved during gameteformation. In the sexual life cycle, there is analternation of diploid and haploid generations.

Chapter 12 Sexual Reproduction and Meiosis 227

Haploid (n)

GametesSperm (n) Egg (n)

Diploid (2n)

Diploid (2n)multicellular organism

Diploid (2n)zygote

Diploid (2n)germ-line cells

Meiosis

Mitosis

Gameteformation

Germ cellformation

Mitosis

Haploid (n) cells

Haploid (n)multicellular organism

Fertilization

FIGURE 12.3Alternation of generations. In sexual reproduction,haploid cells or organisms alternate with diploidcells or organisms.

Male(diploid)

2n

Meiosis

Grows intoadult male oradult female

Sperm(haploid) n

Diploid (2n)

Zygote(diploid) 2n

Fertilization

Female(diploid)

2n

Meiosis

Haploid (n)

Egg(haploid) n

FIGURE 12.4The sexual life cycle. In animals, the completion of meiosis isfollowed soon by fertilization. Thus, the vast majority of the lifecycle is spent in the diploid stage.

Unique Features of MeiosisThe mechanism of cell division varies in important detailsin different organisms. This is particularly true of chromo-somal separation mechanisms, which differ substantially inprotists and fungi from the process in plants and animalsthat we will describe here. Meiosis in a diploid organismconsists of two rounds of division, mitosis of one. Althoughmeiosis and mitosis have much in common, meiosis hasthree unique features: synapsis, homologous recombina-tion, and reduction division.

SynapsisThe first unique feature of meiosis happens early duringthe first nuclear division. Following chromosome replica-tion, homologous chromosomes, or homologues (see chapter 11),pair all along their length. The process of forming thesecomplexes of homologous chromosomes is called synapsis

Homologous RecombinationThe second unique feature of meiosis is that genetic ex-change occurs between the homologous chromosomes while theyare thus physically joined (figure 12.5a). The exchangeprocess that occurs between paired chromosomes is calledcrossing over. Chromosomes are then drawn togetheralong the equatorial plane of the dividing cell; subse-quently, homologues are pulled by microtubules towardopposite poles of the cell. When this process is complete,the cluster of chromosomes at each pole contains one ofthe two homologues of each chromosome. Each pole ishaploid, containing half the number of chromosomes pres-ent in the original diploid cell. Sister chromatids do notseparate from each other in the first nuclear division, soeach homologue is still composed of two chromatids.

Reduction DivisionThe third unique feature of meiosis is that the chromosomesdo not replicate between the two nuclear divisions, so that at theend of meiosis, each cell contains only half the originalcomplement of chromosomes (figure 12.5b). In most re-spects, the second meiotic division is identical to a normalmitotic division. However, because of the crossing overthat occurred during the first division, the sister chromatidsin meiosis II are not identical to each other.

Meiosis is a continuous process, but it is most easily stud-ied when we divide it into arbitrary stages. The stages ofmeiosis are traditionally called meiosis I and meiosis II. Likemitosis, each stage is subdivided further into prophase,metaphase, anaphase, and telophase (figure 12.6). In meio-sis, however, prophase I is more complex than in mitosis.

In meiosis, homologous chromosomes becomeintimately associated and do not replicate between thetwo nuclear divisions.



SYNAPSIS

Homologue Homologue

Region of closeassociation, wherecrossing overoccurs

(a)

Centromere

Sisterchromatids

REDUCTIONDIVISIONDiploid

germ-linecell

Haploid gametes

Chromosomeduplication

Meiosis I

Meiosis II

(b)

FIGURE 12.5Unique features of meiosis. (a) Synapsis draws homologouschromosomes together, creating a situation where the twochromosomes can physically exchange parts, a process calledcrossing over. (b) Reduction division, by omitting a chromosomeduplication before meiosis II, produces haploid gametes, thusensuring that chromosome number remains stable during thereproduction cycle.


Cell division

Celldivision

Celldivision

Synapsis andcrossing over

Pairing ofhomologous chromosomes

Chromosomereplication

Chromosomereplication

Paternal homologue

Maternal homologue

MEIOSIS MITOSIS

ME

IOS

ISI

ME

IOS

ISII

FIGURE 12.6A comparison of meiosis and mitosis. Meiosis involves two nuclear divisions with no DNA replication between them. It thus producesfour daughter cells, each with half the original number of chromosomes. Crossing over occurs in prophase I of meiosis. Mitosis involves asingle nuclear division after DNA replication. It thus produces two daughter cells, each containing the original number of chromosomes.

Prophase IIn prophase I of meiosis, the DNA coils tighter, and indi-vidual chromosomes first become visible under the lightmicroscope as a matrix of fine threads. Because the DNAhas already replicated before the onset of meiosis, each ofthese threads actually consists of two sister chromatidsjoined at their centromeres. In prophase I, homologouschromosomes become closely associated in synapsis, ex-change segments by crossing over, and then separate.

An Overview

Prophase I is traditionally divided into five sequentialstages: leptotene, zygotene, pachytene, diplotene, and dia-kinesis.

Leptotene. Chromosomes condense tightly.Zygotene. A lattice of protein is laid down betweenthe homologous chromosomes in the process of synap-sis, forming a structure called a synaptonemal complex(figure 12.7).

Pachytene. Pachytene begins when synapsis is com-plete (just after the synaptonemal complex forms; figure12.8), and lasts for days. This complex, about 100 nmacross, holds the two replicated chromosomes in preciseregister, keeping each gene directly across from its part-ner on the homologous chromosome, like the teeth of a

zipper. Within the synaptonemal complex, the DNA du-plexes unwind at certain sites, and single strands ofDNA form base-pairs with complementary strands onthe other homologue. The synaptonemal complex thusprovides the structural framework that enables crossingover between the homologous chromosomes. As you


12.3 The sequence of events during meiosis involves two nuclear divisions.

Chromosomehomologues

Synaptonemalcomplex

FIGURE 12.7Structure of the synaptonemal complex. A portion of thesynaptonemal complex of the ascomycete Neotiella rutilans, a cupfungus.

Interphase Leptotene Zygotene Pachytene Diplotene followed by diakinesis

Chromatid 1

Chromatid 2

Chromatid 3

Chromatid 4

Disassemblyof the

synaptonemalcomplex

Formationof the

synaptonemalcomplex

Chromatid 1

Chromatid 2

Chromatid 3

Chromatid 4

Paternalsister

chromatids

Maternalsister

chromatids

Time

Crossing over can occurbetween homologous

chromosomes

FIGURE 12.8Time course of prophase I. The five stages of prophase I represent stages in the formation and subsequent disassembly of thesynaptonemal complex, the protein lattice that holds homologous chromosomes together during synapsis.

will see, this has a key impact onhow the homologues separate laterin meiosis.

Diplotene. At the beginning ofdiplotene, the protein lattice of thesynaptonemal complex disassem-bles. Diplotene is a period of in-tense cell growth. During this pe-riod the chromosomes decondenseand become very active in tran-scription.

Diakinesis. At the beginning ofdiakinesis, the transition intometaphase, transcription ceasesand the chromosomes recondense.

Synapsis

During prophase, the ends of thechromatids attach to the nuclear envelope at specific sites.The sites the homologues attach to are adjacent, so that themembers of each homologous pair of chromosomes arebrought close together. They then line up side by side, ap-parently guided by heterochromatin sequences, in theprocess called synapsis.

Crossing Over

Within the synaptonemal complex, recombination isthought to be carried out during pachytene by very largeprotein assemblies called recombination nodules. A nod-ule’s diameter is about 90 nm, spanning the central elementof the synaptonemal complex. Spaced along the synaptone-mal complex, these recombination nodules act as largemultienzyme “recombination machines,” each nodulebringing about a recombination event. The details of thecrossing over process are not well understood, but involve acomplex series of events in which DNA segments are ex-changed between nonsister or sister chromatids. In hu-mans, an average of two or three such crossover eventsoccur per chromosome pair.

When crossing over is complete, the synaptonemal com-plex breaks down, and the homologous chromosomes arereleased from the nuclear envelope and begin to move awayfrom each other. At this point, there are four chromatidsfor each type of chromosome (two homologous chromo-somes, each of which consists of two sister chromatids).The four chromatids do not separate completely, however,because they are held together in two ways: (1) the two sis-ter chromatids of each homologue, recently created byDNA replication, are held near by their common cen-tromeres; and (2) the paired homologues are held togetherat the points where crossing over occurred within thesynaptonemal complex.

Chiasma Formation

Evidence of crossing over can often be seen under the lightmicroscope as an X-shaped structure known as a chiasma(Greek, “cross”; plural, chiasmata; figure 12.9). The pres-ence of a chiasma indicates that two chromatids (one fromeach homologue) have exchanged parts (figure 12.10). Likesmall rings moving down two strands of rope, the chias-mata move to the end of the chromosome arm as the ho-mologous chromosomes separate.

Synapsis is the close pairing of homologouschromosomes that takes place early in prophase I ofmeiosis. Crossing over occurs between the paired DNAstrands, creating the chromosomal configurationsknown as chiasmata. The two homologues are lockedtogether by these exchanges and they do not disengagereadily.


FIGURE 12.9Chiasmata. This micrograph shows two distinct crossovers, or chiasmata.

FIGURE 12.10The results of crossing over. During crossing over, nonsister(shown above) or sister chromatids may exchange segments.

Metaphase IBy metaphase I, the second stage of meiosis I, the nuclearenvelope has dispersed and the microtubules form a spin-dle, just as in mitosis. During diakinesis of prophase I,the chiasmata move down the paired chromosomes fromtheir original points of crossing over, eventually reachingthe ends of the chromosomes. At this point, they arecalled terminal chiasmata. Terminal chiasmata hold thehomologous chromosomes together in metaphase I, sothat only one side of each centromere faces outward fromthe complex; the other side is turned inward toward theother homologue (figure 12.11). Consequently, spindlemicrotubules are able to attach to kinetochore proteinsonly on the outside of each centromere, and the cen-tromeres of the two homologues attach to microtubulesoriginating from opposite poles. This one-sided attach-ment is in marked contrast to the attachment in mitosis,when kinetochores on both sides of a centromere bind tomicrotubules.

Each joined pair of homologues then lines up on themetaphase plate. The orientation of each pair on the spin-dle axis is random: either the maternal or the paternal ho-mologue may orient toward a given pole (figure 12.12).Figure 12.13 illustrates the alignment of chromosomes dur-ing metaphase I.

Chiasmata play an important role in aligning thechromosomes on the metaphase plate.


Metaphase I

Anaphase I

Meiosis I

Chiasmata

Mitosis

Metaphase

Anaphase

Kinetochores of sisterchromatids remainseparate; microtubulesattach to bothkinetochores onopposite sides of thecentromere.

Microtubules pull sisterchromatids apart.

Chiasmata holdhomologues together.The kinetochores of sister chromatids fuse and function as one. Microtubules can attach to only one side of each centromere.

Microtubules pull thehomologous chromosomes apart, but sister chromatids are held together.

FIGURE 12.11Chiasmata created by crossing over have a key impact on how chromosomes align in metaphase I. In the first meiotic division, thechiasmata hold one sister chromatid to the other sister chromatid; consequently, the spindle microtubules can bind to only one side of eachcentromere, and the homologous chromosomes are drawn to opposite poles. In mitosis, microtubules attach to both sides of eachcentromere; when the microtubules shorten, the sister chromatids are split and drawn to opposite poles.

FIGURE 12.12Random orientation of chromosomes on the metaphase plate. The number of possible chromosome orientations equals 2 raised to the power of the number of chromosome pairs. In thishypothetical cell with three chromosome pairs, eight (23) possible orientations exist, four of them illustrated here. Eachorientation produces gametes with different combinations ofparental chromosomes.


Prophase II

Metaphase IIAnaphase II

Interphase

Prophase I

Meiosis I

Meiosis II

Metaphase I

Anaphase I

Telophase I

Telophase II

FIGURE 12.13The stages of meiosis in alily. Note the arrangementof chromosomes inmetaphase I.

Completing MeiosisAfter the long duration of prophase and metaphase, whichtogether make up 90% or more of the time meiosis I takes,meiosis I rapidly concludes. Anaphase I and telophase Iproceed quickly, followed—without an intervening periodof DNA synthesis—by the second meiotic division.

Anaphase I

In anaphase I, the microtubules of the spindle fibersbegin to shorten. As they shorten, they break the chias-mata and pull the centromeres toward the poles, drag-ging the chromosomes along with them. Because the mi-crotubules are attached to kinetochores on only one sideof each centromere, the individual centromeres are notpulled apart to form two daughter centromeres, as theyare in mitosis. Instead, the entire centromere moves toone pole, taking both sister chromatids with it. When thespindle fibers have fully contracted, each pole has a com-plete haploid set of chromosomes consisting of one mem-ber of each homologous pair. Because of the random ori-entation of homologous chromosomes on the metaphaseplate, a pole may receive either the maternal or the pater-nal homologue from each chromosome pair. As a result,the genes on different chromosomes assort indepen-dently; that is, meiosis I results in the independent as-sortment of maternal and paternal chromosomes intothe gametes.

Telophase I

By the beginning of telophase I, the chromosomes havesegregated into two clusters, one at each pole of the cell.Now the nuclear membrane re-forms around each daugh-ter nucleus. Because each chromosome within a daughternucleus replicated before meiosis I began, each now con-tains two sister chromatids attached by a common cen-tromere. Importantly, the sister chromatids are no longer iden-tical, because of the crossing over that occurred in prophaseI (figure 12.14). Cytokinesis may or may not occur aftertelophase I. The second meiotic division, meiosis II, occursafter an interval of variable length.

The Second Meiotic Division

After a typically brief interphase, in which no DNA synthe-sis occurs, the second meiotic division begins.

Meiosis II resembles a normal mitotic division. ProphaseII, metaphase II, anaphase II, and telophase II follow inquick succession.

Prophase II. At the two poles of the cell the clustersof chromosomes enter a brief prophase II, each nuclearenvelope breaking down as a new spindle forms.

Metaphase II. In metaphase II, spindle fibers bind toboth sides of the centromeres.

Anaphase II. The spindle fibers contract, splitting thecentromeres and moving the sister chromatids to oppo-site poles.

Telophase II. Finally, the nuclear envelope re-formsaround the four sets of daughter chromosomes.

The final result of this division is four cells containinghaploid sets of chromosomes (figure 12.15). No two arealike, because of the crossing over in prophase I. Nuclearenvelopes then form around each haploid set of chromo-somes. The cells that contain these haploid nuclei may de-velop directly into gametes, as they do in animals. Alterna-tively, they may themselves divide mitotically, as they do inplants, fungi, and many protists, eventually producinggreater numbers of gametes or, as in the case of someplants and insects, adult individuals of varying ploidy.

During meiosis I, homologous chromosomes movetoward opposite poles in anaphase I, and individualchromosomes cluster at the two poles in telophase I. Atthe end of meiosis II, each of the four haploid cellscontains one copy of every chromosome in the set,rather than two. Because of crossing over, no two cellsare the same. These haploid cells may develop directlyinto gametes, as in animals, or they may divide bymitosis, as in plants, fungi, and many protists.


FIGURE 12.14After meiosis I, sister chromatids are not identical. So-called“harlequin” chromosomes, each containing one fluorescent DNAstrand, illustrate the reciprocal exchange of genetic materialduring meiosis I between sister chromatids.


MEIOSIS

Germ-line cell

Haploid gametes

PROPHASE

I

IITELOPHASE

I IIANAPHASE

II

I

II I

METAPHASE

FIGURE 12.15How meiosis works. Meiosis consists of two rounds of cell division and produces four haploid cells.

Why Sex?Not all reproduction is sexual. In asexual reproduction,an individual inherits all of its chromosomes from a sin-gle parent and is, therefore, genetically identical to itsparent. Bacterial cells reproduce asexually, undergoingbinary fission to produce two daughter cells containingthe same genetic information. Most protists reproduceasexually except under conditions of stress; then theyswitch to sexual reproduction. Among plants, asexual re-production is common, and many other multicellular or-ganisms are also capable of reproducing asexually. In ani-mals, asexual reproduction often involves the budding offof a localized mass of cells, which grows by mitosis toform a new individual.

Even when meiosis and the production of gametesoccur, there may still be reproduction without sex. Thedevelopment of an adult from an unfertilized egg, calledparthenogenesis, is a common form of reproduction inarthropods. Among bees, for example, fertilized eggs de-velop into diploid females, but unfertilized eggs developinto haploid males. Parthenogenesis even occurs amongthe vertebrates. Some lizards, fishes, and amphibians arecapable of reproducing in this way; their unfertilized eggsundergo a mitotic nuclear division without cell cleavageto produce a diploid cell, which then develops into anadult.

Recombination Can Be Destructive

If reproduction can occur without sex, why does sex occurat all? This question has generated considerable discussion,particularly among evolutionary biologists. Sex is of greatevolutionary advantage for populations or species, whichbenefit from the variability generated in meiosis by randomorientation of chromosomes and by crossing over. How-ever, evolution occurs because of changes at the level of in-dividual survival and reproduction, rather than at the popu-lation level, and no obvious advantage accrues to theprogeny of an individual that engages in sexual reproduc-tion. In fact, recombination is a destructive as well as a con-structive process in evolution. The segregation of chromo-somes during meiosis tends to disrupt advantageouscombinations of genes more often than it creates new, bet-ter adapted combinations; as a result, some of the diverseprogeny produced by sexual reproduction will not be aswell adapted as their parents were. In fact, the more com-plex the adaptation of an individual organism, the less likelythat recombination will improve it, and the more likely thatrecombination will disrupt it. It is, therefore, a puzzle toknow what a well-adapted individual gains from participat-ing in sexual reproduction, as all of its progeny could main-tain its successful gene combinations if that individual sim-ply reproduced asexually.

The Origin and Maintenance of Sex

There is no consensus among evolutionary biologists re-garding the evolutionary origin or maintenance of sex.Conflicting hypotheses abound. Alternative hypothesesseem to be correct to varying degrees in differentorganisms.

The DNA Repair Hypothesis. If recombination is oftendetrimental to an individual’s progeny, then what benefitpromoted the evolution of sexual reproduction? Althoughthe answer to this question is unknown, we can gain someinsight by examining the protists. Meiotic recombination isoften absent among the protists, which typically undergosexual reproduction only occasionally. Often the fusion oftwo haploid cells occurs only under stress, creating adiploid zygote.

Why do some protists form a diploid cell in responseto stress? Several geneticists have suggested that this oc-curs because only a diploid cell can effectively repair cer-tain kinds of chromosome damage, particularly double-strand breaks in DNA. Both radiation and chemicalevents within cells can induce such breaks. As organismsbecame larger and longer-lived, it must have become in-creasingly important for them to be able to repair suchdamage. The synaptonemal complex, which in early stagesof meiosis precisely aligns pairs of homologous chromo-somes, may well have evolved originally as a mechanismfor repairing double-strand damage to DNA, using theundamaged homologous chromosome as a template to re-pair the damaged chromosome. A transient diploid phasewould have provided an opportunity for such repair. Inyeast, mutations that inactivate the repair system for dou-ble-strand breaks of the chromosomes also prevent cross-ing over, suggesting a common mechanism for bothsynapsis and repair processes.

The Contagion Hypothesis. An unusual and interestingalternative hypothesis for the origin of sex is that it arose asa secondary consequence of the infection of eukaryotes bymobile genetic elements. Suppose a replicating transpos-able element were to infect a eukaryotic lineage. If it pos-sessed genes promoting fusion with uninfected cells andsynapsis, the transposable element could readily copy itselfonto homologous chromosomes. It would rapidly spread byinfection through the population, until all members con-tained it. The bizarre mating type “alleles” found in manyfungi are very nicely explained by this hypothesis. Each ofseveral mating types is in fact not an allele but an “id-iomorph.” Idiomorphs are genes occupying homologouspositions on the chromosome but having such dissimilarsequences that they cannot be of homologous origin. Theseidiomorph genes may simply be the relics of several ancientinfections by transposable elements.



The Red Queen Hypothesis. One evolutionary ad-vantage of sex may be that it allows populations to“store” recessive alleles that are currently bad but havepromise for reuse at some time in the future. Becausepopulations are constrained by a changing physical andbiological environment, selection is constantly actingagainst such alleles, but in sexual species can never get ridof those sheltered in heterozygotes. The evolution ofmost sexual species, most of the time, thus manages tokeep pace with ever-changing physical and biologicalconstraints. This “treadmill evolution” is sometimescalled the “Red Queen hypothesis,” after the Queen ofHearts in Lewis Carroll’s Through the Looking Glass, whotells Alice, “Now, here, you see, it takes all the runningyou can do, to keep in the same place.”

Miller’s Ratchet. The geneticist Herman Miller pointedout in 1965 that asexual populations incorporate a kind ofmutational ratchet mechanism—once harmful mutationsarise, asexual populations have no way of eliminating them,and they accumulate over time, like turning a ratchet. Sex-ual populations, on the other hand, can employ recombina-tion to generate individuals carrying fewer mutations,which selection can then favor. Sex may just be a way tokeep the mutational load down.

The Evolutionary Consequences of Sex

While our knowledge of how sex evolved is sketchy, it isabundantly clear that sexual reproduction has an enormousimpact on how species evolve today, because of its ability torapidly generate new genetic combinations. Independentassortment (figure 12.16), crossing over, and random fertil-ization each help generate genetic diversity.

Whatever the forces that led to sexual reproduction, itsevolutionary consequences have been profound. No genetic

process generates diversity more quickly; and, as you willsee in later chapters, genetic diversity is the raw material ofevolution, the fuel that drives it and determines its poten-tial directions. In many cases, the pace of evolution appearsto increase as the level of genetic diversity increases. Pro-grams for selecting larger stature in domesticated animalssuch as cattle and sheep, for example, proceed rapidly atfirst, but then slow as the existing genetic combinations areexhausted; further progress must then await the generationof new gene combinations. Racehorse breeding provides agraphic example: thoroughbred racehorses are all descen-dants of a small initial number of individuals, and selectionfor speed has accomplished all it can with this limitedamount of genetic variability—the winning times in majorraces ceased to improve decades ago.

Paradoxically, the evolutionary process is thus bothrevolutionary and conservative. It is revolutionary in thatthe pace of evolutionary change is quickened by geneticrecombination, much of which results from sexual repro-duction. It is conservative in that evolutionary change isnot always favored by selection, which may instead pre-serve existing combinations of genes. These conservativepressures appear to be greatest in some asexually repro-ducing organisms that do not move around freely andthat live in especially demanding habitats. In vertebrates,on the other hand, the evolutionary premium appears tohave been on versatility, and sexual reproduction is thepredominant mode of reproduction by an overwhelmingmargin.

The close association between homologouschromosomes that occurs during meiosis may haveevolved as mechanisms to repair chromosomal damage,although several alternative mechanisms have also beenproposed.


Paternal gamete

Diploid offspring

Maternal gamete

Homologous pairs

Potential gametes

FIGURE 12.16Independent assortment increases genetic variability. Independent assortment contributes new gene combinations to the nextgeneration because the orientation of chromosomes on the metaphase plate is random. In the cells shown above with three chromosomepairs, eight different gametes can result, each with different combinations of parental chromosomes.


Chapter 12Summary Questions Media Resources


• Meiosis is a special form of nuclear division thatproduces the gametes of the sexual cycle. It involvestwo chromosome separations but only onechromosome replication.

1. What are the cellular productsof meiosis called, and are theyhaploid or diploid? What is thecellular product of syngamycalled, and is it haploid ordiploid?

• The three unique features of meiosis are synapsis,homologous recombination, and reduction division.

2. What three unique featuresdistinguish meiosis from mitosis?


• The crossing over that occurs between homologuesduring synapsis is an essential element of meiosis.

• Because crossing over binds the homologuestogether, only one side of each homologue isaccessible to the spindle fibers. Hence, the spindlefibers separate the paired homologues rather than thesister chromatids.

• At the end of meiosis I, one homologue of eachchromosome type is present at each of the two poles ofthe dividing nucleus. The homologues still consist oftwo chromatids, which may differ from each other as aresult of crossing over that occurred during synapsis.

• No further DNA replication occurs before the secondnuclear division, which is essentially a mitotic divisionoccurring at each of the two poles.

• The sister chromatids of each chromosome areseparated, resulting in the formation of four daughternuclei, each with half the number of chromosomesthat were present before meiosis.

• Cytokinesis typically but not always occurs at thispoint. When it does, each daughter nucleus has onecopy of every chromosome.

3. What are synaptonemalcomplexes? How do theyparticipate in crossing over? Atwhat stage during meiosis arethey formed?4. How many chromatids arepresent for each type ofchromosome at the completionof crossing over? What twostructures hold the chromatidstogether at this stage?5. How is the attachment ofspindle microtubules tocentromeres in metaphase I ofmeiosis different from thatwhich occurs in metaphase ofmitosis? What effect does thisdifference have on themovement of chromosomesduring anaphase I?6. What mechanism isresponsible for the independentassortment of chromosomes?

12.3 The sequence of events during meiosis involves two nuclear divisions.

• In asexual reproduction, mitosis produces offspringgenetically identical to the parent.

• Meiosis is thought to have evolved initially as amechanism to repair double-strand breaks in DNA,in which the broken chromosome is paired with itshomologue while it is being repaired.

• The evolutionary significance of meiosis is that itgenerates large amounts of recombination, rapidlyreshuffling gene combinations, producing variabilityupon which evolutionary processes can act.

7. What is one of the currentscientific explanations for theevolution of synapsis?8. By what three mechanismsdoes sexual reproductionincrease genetic variability? Howdoes this increase in geneticvariability affect the evolution ofspecies?


http://www.mhhe.com/raven6e http://www.biocourse.com

• Art Activity: Meiosis I

• Meiosis

• Meiosis

• Evolution of Sex• Review of Cell

Division

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Chapter 12: Sexual Reproduction and Meiosis

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