TERM PAPER PRESENTATION ON

DEVELOPMENTAL HOMEOSTASIS AND HOMOZYGOSITY

GOWTHAMI R

HOMEOSTASIS

Homeostasis refers to the property of the organism to adjust itself to variable conditions or self regulation (genetic, physiological or morphological) of populations of plants and animals against any fluctuating and sudden change in the external environment.

The concept of self regulation was first suggested by CLAUDE BERNARD Inin 1865.

The term homeosstasis was given by CANNON in 1932

TYPES OF HOMEOSTASIS

LERNER in 1954 distinguished two types of homeostasis

1. GENETIC HOMEOSTASIS

2. DEVELOPMENTAL HOMEOSTASIS

GENETIC HOMEOSTASIS- is the property of a population of genotypes to resist sudden changes.

DEVELOPMENTAL HOMEOSTASIS- The ability of Individual genotypes to adopt themselves to variable environments and to produce predictable phenotypes

The above terms were also referred as populational homeostasis and individual homeostasis by LEWONTIN in 1957

Populational buffering and individual buffering by ALLARD AND BRADSHAW in 1964

Heterozygous or heterogeneous individuals are more homeostatic than homozygous or homogeneous individuals.

Homeostasis was inversly proportional to the variability expressed by the individuals

OUTCOMES OF DEVELOPMENTAL HOMEOSTASIS

1. CANALIZATION- is the ability of a genotype to express the same

phenotype across environments.

2. DEVELOPMENTAL STABILITY- The repeatability of the same

character within a specific environment.

QUANTIFYING DEVELOPMENTAL HOMEOSTASIS

Fluctuating assymetry

Extent to which a trait departs from the

normal individuals

VARIATION OF EXPRESSION OF DEVELOPMENTAL HOMEOSTASIS

1. EMIPRICAL EVIDENCE :- If the evolution of developmental homeostasis for traits follows a NEO-DARWINISM process there must be a variation in expression among individuals

2. GENETIC STRESS :- Mutations reduces developmental homeostasis.

Inbred lines decreases developmental homeostasis. Hybridisation among related species also results to reduce developmental homeostasis.

3. ENVIRONMENTAL STRESS :- Environmental factors, Anthropogenic sources of pollution

GENETIC BASIS OF DEVELOPMENTAL HOMEOSTASIS

The contribution of genetic variation to a trait can be partitioned into further components reflecting the different modes of gene action i.e.

a. ADDITIVE COMPONENT

b. DOMINANCE COMPONENT

c. INTERACTION OR EPISTATIC COMPONENT

ADDITIVE GENETIC VARIATION

The resemblance between parents & offspring is due to additive genetic variation

Selection acting on differences among individuals results in evolutionary response depending on the degree to which additive genetic variation underlies phenotypic variation.

DOMINANCE OR HETEROZYGOSITY

The role of heterozygosity in developmental homeostasis was first emphasized by LERNER (1954) and it was termed as PHENODEVIANTS to describe individuals whose development is more prone to environment fluctuations.

LERNER used an overdominance hypothesis model to describe the relationship between heterozygosity and developmental homeostasis.

EPISTASIS OR COADAPTED GENE COMPLEXES

Dobzhansky (1970) first suggested that coadapted genes complexes are responsible for the expression of developmental homeostasis.

In natural or artificial hybrid populations, coadapted gene complexes may be by the mixing of the parental genomes. Therefore, hybrid studies have been used to demonstrate the presence of coadapted genes.

GENETIC MODELS OF DEVELOPMENTAL HOMEOSTASIS

1. CROW-MULLER MODEL

2. DOZHANSKY MODEL

3. LERNERS MODEL

CROW-MULLER MODEL

Heterosis historically pertains to the vigor seen in hybrids, it is sometimes used interchangeably in the literature with developmental homeostasis in the sense that hybrids who manifest true heterosis will also manifest developmental homeostasis.

High Darwinian fitness is associated with developmental homeostasis.

Most individuals in a Mendelian population should, then, be homozygous for most genes. Heterozygous loci will be a minority.”

Heterozygosity in a population occurs because of

(1) Recurrent mutation at each locus coupled with the interaction of selection,

(2) The presence of genetic variants which are adaptively neutral, or which possess slight adaptive advantages at some times in some places,

(3) Adaptive polymorphism maintained by the diversity of habitats occupied by the population, and

4) the rare good alleles which have not had time to displace their alleles.

DOBZHANSKY MODEL- 1950

Importance of coadapted in the evolutionary process and for heterosis (developmental homeostasis).

Dobzhansky’s often cited 1950 paper emphasizing the importance of coadapted heterozygosity in the evolutionary process and his prior publications promoting neoDarwinism,

The term genomic coadaptation can be defined, therefore, as the coadapted interactions of the genes in the genome or genomes present in an organism.

The term coadaptation used by Dobzhansky, Lerner, and certain other biologists, refers to the mutual adjustmentsthat occur in a gene pool by the action of natural selection to produce desirable outcomes.

According to Lerner (1959) the process of coadaptation is probably a continuous one. A change in allelic frequencies at one locus by selection or by some other force results in a change of allelic frequencies atother loci.

He proposed there is selection for mutations at loci within the region of each inversion that result in overdominance (i.e., the heterozygote may be more fit then either homozygote because of the greater biochemical versatility of having the products of two alleles rather than the products of the same allele).

Mendelian populations, by the action of natural selection, acquire a high level of obligate heterozygosity at loci in polygenic systems, which become important genetic mechanisms for developmental homeostasis.

Individuals who are highly heterozygous at the loci comprising these multigenic (polygenic) systems are buffered during development, and individuals who are highly homozygous at these loci may manifest phenotypicexpressions consistent with lack of canalization.

LERNERS MODEL

IMPORTANCE OF DH

DH provide a way to understand how a complex traits are evolved.

The breakdown of DH is necessary for population to release variability and to respond to new selection pressure.

Now a days more applicable in plants also because

1. It is easy to quantify the relationship between DH and fitness

2. The modular construction in plants indicates that the DH may differ among the levels of organisation within plant

3. Great range of mating and genetic systems found among plant species helps in quantfying DH

CASE STUDY- ESTIMATES OF GENETIC HOMEOSTASIS IN MAIZE

Objective- to estimate genetic homeostasis in maize by using environmental variance

Materials- P1,P2,F2,F3,BC1,BC2,single crosses

Results- the homogenous inbred lines and single cross entries had more GXE interaction mean squares than heterogeneous F2, F3, BC1, BC2

Inbred lines were less homeostatic than the single crosses and F2, and backcross generations were most homeostatic populations

HETEROZYGOTE SUPERIORITY AS DISCUSSED BY LERNER 1954 IS THE MAJOR EXPLANATION FOR BETTER HOMEOSTASIS IN POPULATIONS CONTAINING HIGHER LEVELS OF HETEROZYGOSITY OVER THOSSE WITH NONE OR REDUCED LEVELS OF HETEROZYGOSITY