Sex is determined by genetic constitution (GSD) in most animals, such as by the sex chromosome pairs XX or XY in humans, or ZZ or ZW in birds. In certain other animals, however, sex is determined by environmental temperature (TSD). Among turtle species, however, examples of both types of sex determination can be found. It remains unclear, however, how and why different sex determining mechanisms exist. This collaborative project will help fill this gap by comparing the chromosomes and sex-related genes of turtles using XX/XY, ZZ/ZW, or TSD mechanisms for sex determination. Particularly, this research will test whether (1) all sex chromosomes in turtles derive from a common ancestral pair of non-sex chromosomes (or "autosomes"), and whether (2) the molecular evolution of genes located on turtle sex chromosomes differs from that of the same genes in TSD turtles, in which there are no sex chromosomes- i.e., whether the sequences of genes located on sex chromosomes change differently over evolutionary time than do the sequences of the same genes when they are located on autosomes.
This project will integrate undergraduate discovery-based learning for undergraduates into the field and lab modules of this project and will broaden the participation of underrepresented groups in biology by providing direct training opportunities for students and their mentors, by engaging women and minorities through outreach activities, and by providing further opportunities for both the investigators to serve as role models for minorities.
Nicole Valenzuela and Scott V. Edwards MCB 0815354 Sex Chromosome Evolution in Turtles Sex is determined by genetic constitution in most animals (as by sex XX/XY chromosomes in humans, or ZZ/ZW in birds), and by environmental temperature (TSD) in others, including turtles. Although the association of sex chromosomes with the development of males and females is well established, it remains unclear how and why different sex determining mechanisms exist. This collaborative project helped fill this gap by discovering novel XX/XY and ZZ/ZW sex chromosome system in several turtle species previously thought to have undistinguishable sex chromosome pairs, by studying the molecular evolution and chromosomal location of genes involved in sexual development across turtles with contrasting mechanisms of sex determination, by characterizing important events in the evolution of chromosomes in several lineages of turtles, and by developing new genomic resources to aid further studies. In particular, results from this project show that a ZZ/ZW mechanism evolved in the Tryonichidae turtle family ~95mya and has remained remarkably unchanged since, while a striking morphological divergence is observed among younger XX/XY systems in the Chelidae turtle family. These findings highlight the need to understand the drivers behind sex chromosome lability and conservation in different lineages. Additionally, turtle lineages where transitions between sex determination modes occur, exhibit a ~20-fold higher rate of evolution of chromosome number. These data indicate that the evolution of sex determination is associated with profound genomic changes, beyond the evolution of sex chromosomes and live bearing. Intellectual Merit — This project advances our understanding of the enigmatic evolution of sex chromosomes and sex-linked genes using explicit hypotheses testing about the evolutionary transitions of sex chromosomes and their consequences at the gene and genome level. Our data appear to counter the recent provocative hypothesis that all vertebrate sex chromosomes derived from a single common ancestor. Our findings reveal strong interactions across levels of biological organization (genome structure, development, ecology) – which can fuel co-evolutionary dynamics, and underscore the potential evolutionary consequences of current climate change. Broader Impact — This project provided training opportunities and integrated undergraduate discovery-based learning for undergraduate students and K-12 teachers through the Iowa Turtle Army (ITA) program developed by the PI plus internships in the Co-PI’s lab. During the tenure of this grant, 25 undergraduate students and 2 K-12 teachers participated into the field and lab modules in the PI’s lab. ITA interns learned about genomics, field, experimental and developmental biology, herpetology, evolutionary and molecular ecology, and classic and molecular cytogenetics. This project also provided training and development opportunities to 4 graduate and 2 postdoctoral students in research and mentoring, plus research opportunities for 1 undergrad and 2 graduate international students. Additionally, this project helped broaden the participation of underrepresented groups in biology by providing direct training opportunities for underrepresented students (21 female, 3 minority); and by engaging women and minorities through outreach activities such as the Program for Women in Science and Engineering and the George Washington Carver Summer Program, among others. RET K-12 teachers developed curricular material during their summer internships with the ITA, including posters and lesson plans. It is intended that this material be shared for their use by peer teachers and institutions, and can be downloaded from the ITA’s website.
