REFINING THE LINKAGE MAP OF THE CHICKEN Z CHROMOSOME J. J. BITGOOD, USA Department of Poultry Science, University of Wisconsin-Madison, Madison, WI 53706 SUMMARY In the last ten years, additional genetic markers in the form of chromosome rearrangements have become available for use in expanding the genetic linkage map of the chicken. A series of studies that have been conducted by several laboratories have made it possible to identify the long and short arms of the nearly metacentric chicken Z chromosome. Breakpoints of several chromosome translocations have been mapped against selected phenotypic traits. Three new Z-linked recessive mutations are being mapped against known loci and chromosome rearrangement breakpoints. These mutations are pop-eye, assigned to the short arm, and recessive white skin and restricted ovulator, both of which appear to be located on the long arm. Further tests are needed to assign these loci to positions in the linear map, and some tests are underway. A tester line that will be recessive for several Z-linked mutations is being developed. INTRODUCTION The genetic linkage map of the chicken Z chromosome is probably the best known sex chromosome map of any economically important farm animal. Spillman (1908) first suggested the sex-linked mode of inheritance for barring (B). Later, Haldane (1921) reported linkage of 34 map units between J3 and silver (j[), the first report of linkage of loci on the Z chromosome. It has since been shown that these loci lie far enough apart on the Z chromosome that they will segregate independently. Development of the map as currently published (Somes, 1984) was accomplished by use of classical Mendelian genetic analyses. Beginning with Zartman's (1973) report assigning pea comb to chromosome 1 by use of two chromosome translocations, cytogenetic methods have provided additional genetic markers for linkage studies. More recently, molecular biology has provided additional information for the localization of genetic markers on the Z chromosome. Combining different techniques in investigations of the chicken Z chromosome is resulting in a more refined genetic map than any one procedure could accomplish. RECENT FINDINGS In the last ten years, use of chromosome rearrangements has allowed assignment of loci to specific arms of the Z chromosome. Telloni et al. (1976) were the first investigators to use a Z-linked translocation in a linkage study of the Z chromosome. They reported that the breakpoint of a t(Z;micro) rearrangement was 23 map units from the Z-linked rate of feathering (K) locus. They also reported that 15 did not show linkage with the breakpoint. Bitgood et al. (1980) used the MN t(Z;l) (Wang e_t al., 1982) and the NM 7092 t(Z;l) (Zartman, 1973) rearrangements in linkage studies of B and j>. 15 was found to lie 22 map units from the MN t(Z;l), and S_ was found to be 6 map units from the NM 7092 t(Z;l). Bitgood (1980) conducted a G-band analysis of these two translocations and demonstrated that the breakpoints were on opposite arms. The MN t(Z;l) breakpoint was on the arm of the Z chromosome that was marked by the large light staining region, and the NM 7092 t(Z;l) was on the opposite arm. These findings indicated that B and showed independent assortment because they were, in fact, located a good distance 289
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REFINING THE LINKAGE MAP OF THE CHICKEN Z CHROMOSOME
J. J. BITGOOD, USA Department of Poultry Science,
University of Wisconsin-Madison, Madison, WI 53706SUMMARY
In the last ten years, additional genetic markers in the form of chromosome rearrangements have become available for use in expanding the genetic linkage map of the chicken. A series of studies that have been conducted by several laboratories have made it possible to identify the long and short arms of the nearly metacentric chicken Z chromosome. Breakpoints of several chromosome translocations have been mapped against selected phenotypic traits. Three new Z-linked recessive mutations are being mapped against known loci and chromosome rearrangement breakpoints. These mutations are pop-eye, assigned to the short arm, and recessive white skin and restricted ovulator, both of which appear to be located on the long arm. Further tests are needed to assign these loci to positions in the linear map, and some tests are underway. A tester line that will be recessive for several Z-linked mutations is being developed.
INTRODUCTION
The genetic linkage map of the chicken Z chromosome is probably the best known sex chromosome map of any economically important farm animal. Spillman (1908) first suggested the sex-linked mode of inheritance for barring (B). Later, Haldane (1921) reported linkage of 34 map units between J3 and silver (j[), the first report of linkage of loci on the Z chromosome. It has since been shown that these loci lie far enough apart on the Z chromosome that they will segregate independently. Development of the map as currently published (Somes, 1984) was accomplished by use of classical Mendelian genetic analyses. Beginning with Zartman's (1973) report assigning pea comb to chromosome 1 by use of two chromosome translocations, cytogenetic methods have provided additional genetic markers for linkage studies. More recently, molecular biology has provided additional information for the localization of genetic markers on the Z chromosome. Combining different techniques in investigations of the chicken Z chromosome is resulting in a more refined genetic map than any one procedure could accomplish.
RECENT FINDINGS
In the last ten years, use of chromosome rearrangements has allowed assignment of loci to specific arms of the Z chromosome. Telloni et al.(1976) were the first investigators to use a Z-linked translocation in a linkage study of the Z chromosome. They reported that the breakpoint of a t(Z;micro) rearrangement was 23 map units from the Z-linked rate of feathering (K) locus. They also reported that 15 did not show linkage with the breakpoint. Bitgood et al. (1980) used the MN t(Z;l) (Wang e_t al., 1982) and the NM 7092 t(Z;l) (Zartman, 1973) rearrangements in linkage studies of B and j>. 15 was found to lie 22 map units from the MN t(Z;l), and S_ was found to be6 map units from the NM 7092 t(Z;l). Bitgood (1980) conducted a G-band analysis of these two translocations and demonstrated that the breakpoints were on opposite arms. The MN t(Z;l) breakpoint was on the arm of the Z chromosome that was marked by the large light staining region, and the NM 7092 t(Z;l) was on the opposite arm. These findings indicated that B and showed independent assortment because they were, in fact, located a good distance
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apart on opposite arms of the chromosome.
Kaelbling and Fechheimer (1983a;b) examined synaptonemal complexes (SC's) in chicken spermatozoa. Their measurements of SC's formed by carriers of the same t(Z;micro) rearrangement studied by Telloni nt al. (1976) showed that the breakpoint, and K (based on the earlier study), were both on the short arm of the Z chromosome. As K is closely linked to jS, and Bitgood e_t al. (1980) had shown that the NM 7092 t(Z;l) and £ were closely linked, this translocation breakpoint is also on the short arm of the Z chromosome. Further, as Bitgood (1980) had shown that the large light staining G-band region was on the intact arm in the NM 7092 t(Z;l) rearrangement, this light staining region marks the long arm of the Z chromosome.
Wang and Shoffner (1974) conducted a G-band analysis of the MN t(Z;3) translocation. Their results showed that the breakpoint in this translocation was on the chromosome arm that did not contain the light staining region, or the short arm. This translocation was later tested against S_ (Bitgood,1985b), and no recombinants were recovered among 217 progeny. In a later test, this same rearrangement was tested in a 3 point test cross against both S and K (which lie 1.1 map units apart) in an attempt to determine the linear order of these three markers. Again, no recombinants were recovered among 232 progeny (Bitgood, unpublished).
Another recently reported mutation, pop eye, (pop) has been assigned to the short arm of the Z chromosome (Bitgood and Whitley, 1985). It lies 35 map units from K, and 22 map units from the NM 7092 t(Z;l) translocation. As the NM 7092 t(Z;l) lies 6 maps units from £ (S being closely linked to K), it would appear that the linear order on this arm of the chromosome is (£> ,K,) - NM 7092 - pop, with the order of S_ and K not clear at this time. The direction of the centromere from this region is also unknown.
Kaelbling and Fechheimer (1985b) also demonstrated that the breakpoint on the Z chromosome in the NM 7659 t(Z;l) translocation (Zartman, 1973) was on the short arm. Bitgood (1985a) tested this same translocation against the Z-linked dermal melanin locus (id+) and found that these markers segregated independently. As id+ is linked with B, and 15 is on the opposite long arm, these results are consistent.
Currently, the MN t(Z;l) translocation is the only translocation that has been reported to have the breakpoint on the long arm of the Z chromosome. Bitgood et al. (1980) located 13 22 map units from this breakpoint. Bitgood (1985b) also reported 18 map units between this breakpoint and Z-linked recessive white skin (y) (McGibbon, 1981). IB and y were also tested against each other, and these loci were found to segregate independently. Therefore, the breakpoint of the MN t(Z;l) rearrangement lies between these two loci.
Some recent work (Bitgood, unpublished) also suggests that the Z-linked restricted ovulator mutation (ro) (Jones e_t al., 1975) is located on the long arm of the Z chromosome. A test cross between y_ and ro resulted in approximately 32% recombination between these loci.
Several studies using molecular approaches have located viral loci on the chicken Z chromosome. Tereba et al. (1981) have localized an endogenous retrovirus locus, ev7, near the end of one arm of the chromosome. Bacon et al. (1985) have reported that the endogenous virus ev21 is either very closely linked to the K locus, or else slow feathering is caused by
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integration of the ev21 virus in or near the K locus.
DISCUSSION
Results from a series of cytogenetic studies now allow specific identification of the long and short arms of the Z chromosome. These studies have also provided further information on linkage relationships within the Z chromosome. One interesting finding is the independent segregation exhibited by and j, even though both loci are apparently located on the same arm of the chromosome. This is indicative of the long genetic length of the Z chromosome, which is the fifth longest chromosome in the chicken. Chromosomes 1 through 4 are expected to show even greater genetic length, which will make linkage mapping even more difficult.
Further studies in progress will refine the genetic map of the chicken Z chromosome. A test between B, £ and the Z-linked dermal melanin locus (Id) is in progress to order these loci. Eventually the location of the centromere in relation to the available marker loci will be determined. One method that was used by Bitgood (1985a) to map the centromere of chromosome 1 utilized a combination of cytogenetics and classical Mendellan linkage studies.
Tester stocks are being developed that will allow multiple loci testing. The ro, and pop mutants were discovered in, and had been maintained in White Leghorn lines. These are being placed into genetic backgrounds that will allow more detailed testing against other Z-linked loci, such as B andS.. These recessive tester stocks will also be useful in testing specific, marked segments of the Z chromosome for their effects on economic traits.
Knowledge of the genetic content and behavior of the Z chromosome of the chicken is important because of the unique inheritance pattern of this chromosome and its genetic material. Due to the heterogametic nature of the avian female, her Z chromosome complement is immediately evident, whether the trait in question is dominant or recessive. This has important consequences in applied poultry breeding programs. The use of the rate of feathering locus (K;k+) for sexing egg production stocks in the hatchery, and the interest in the use of the dwarfing gene in females of both egg production strains and broiler breeder stocks are two examples of current commercial utilization of the genetic uniqueness of the Z chromosome. As techniques in molecular biology are developed further and procedures refined, a comprehensive linkage map will assist in identifying and isolating chromosomal segments containing DNA sequences of potential interest.
REFERENCES
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