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Abstract of Daniel Janes' NIH Postdoctoral Fellowship


Evolution of sex chromosomes and sex-determining genes in reptiles

 The major questions associated with this effort are:

1) Has the heteromorphic pair of sex chromosomes evolved from the same or different pair of autosomes within different reptile lineages?

2) Are sex-determining (SD) genes and gene clusters organized in a similar fashion in reptiles as in birds and mammals?

3) What is the organization of SD genes in reptiles exhibiting ESD?

4) Do sex chromosomes in reptiles show increased rates of evolution and decreased rates of recombination relative to autosomes, as has been found in mammals and some birds?

To address these questions, I will screen BAC libraries of alligators and emus for SD genes, and sequence these genes and their surrounding genomic neighborhoods using shotgun methods. I will map these genes on BAC clones to chromosomes prepared from tissue culture. Also, I will compare sequences of Dax1 , Dmrt1, Asw, Sox9, Sox3 , and Sry from American alligators, emus, painted turtles, tuataras, chickens, and humans for phylogenetic analysis and evaluation of molecular clocks. All necessary BAC libraries are available in Dr. Scott Edwards' laboratory at the Museum of Comparative Zoology Laboratories , Harvard University . Additional tools and training will be provided by Dr. J.A.M. Graves at the Australian National University , Canberra , during two 4-month visits to her laboratory.



Although almost all chordates share the fundamental trait of sexuality, the manner in which two sexes are maintained genetically is not clearly understood and may vary considerably among taxa. By comparing the composition of genes on chromosomes, we can draw inferences about the evolutionary divergence between chromosomes and among species that possess those chromosomes (Jegalian and Page 1998). Ohno (1967) theorized that Z and W chromosomes arose from the same pair of autosomes from which the X and Y chromosomes arose. More recent evidence suggests completely independent origins of ZZ/ZW chromosome pairs and XX/XY chromosome pairs. For example, the chicken Z chromosome is derived from six different human autosomal arms, not the human X chromosome (Fridolfsson et al. 1998). It is still not known whether independent origin of sex chromosomes in major clades of vertebrates is the rule or exception.



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