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Studying Hybridisation, ploidy dynamics and reproductive plasticity in whiptail lizards

Supervisor: Peter Baumann

Co-Supervisor: Romain Libbrecht


Scientific Background:
Interspecific hybridization has long been recognized as a key driver in speciation in plants, but its role in animal evolution is still underappreciated, even though recent work has revealed evidence of hybridization events in the history of our own species. In the lizard genus Aspidoscelis, interspecific hybridization played a key role in the transition from sexual to parthenogenetic reproduction, and the resulting high level of heterozygosity is often cited as causal for the evolutionary success of parthenogenetic lineages. Secondary hybridization events increased ploidy and further expanded the genetic repertoire that may permit adaptation to a changing environment or the exploitation of new ecological niches. Indeed, two thirds of parthenogenetic species of whiptail lizards are triploids, whereas some of the diploid intermediates are believed to be extinct. Taking advantage of a laboratory colony, we have produced over 100 hybrids of parthenogenetic females of A. exsanguis, A. uniparens and A. sonorae with males of various sexually reproducing species. These experiments resulted in the several hybrid lizards that have founded new genetically isolated lineages1-3. This provides a unique opportunity to study gene regulatory changes that ensue when the genomes from different species are brought together during hybridization and are subsequently maintained from generation to generation in the absence of recombination. 


PhD project: Studying Hybridisation, ploidy dynamics and reproductive plasticity in whiptail lizards
Here we propose to examine the effects of hybridization on transcriptional regulation, transposable elements and their control by piRNAs over the initial 10 generations of clonal reproduction. We will also examine the effect of secondary and tertiary hybridization events on fertility and test whether a switch from parthenogenetic to sexual reproduction can occur. Each hybridization event generates male and female offspring at a 1:1 ratio. We have found that certain species combinations produce sterile males and females, whereas others produce females with unimpaired ability to reproduce parthenogenetically and males that produce mature sperm. This has opened up a new angle for investigating causes for hybrid sterility as well as the molecular basis for the switch from bisexual to parthenogenetic reproduction and the possibility of a reverse switch involving males that carry the genetic information of a parthenogenetic species. Taking advantage of extensive resources generated by our group, we will use cytological, molecular and genomic approaches to decipher the elements that block parthenogenesis in some hybrids but not others. Romain Libbrecht will contribute his expertise in evolutionary genomics and gene regulation in parthenogenetic insects. This project will further benefit from the expertise of Rene Ketting on piRNAs, Joan Barau on transposons, Claudia Keller Valsecchi on sex determination and dosage compensation.


Publications relevant to this project
1 Cole, C. J. et al. The Second Known Tetraploid Species of Parthenogenetic Tetrapod (Reptilia: Squamata: Teiidae): Description, Reproduction, Comparisons with Ancestral Taxa, and Origins of Multiple Clones. Bulletin of the Museum of Comparative Zoology 161, 285-321 (2017).
2 Cole, C. J., Taylor, H. L., Baumann, D. P. & Baumann, P. Neaves' whiptail lizard: the first known tetraploid parthenogenetic tetrapod (Reptilia: Squamata: Teiidae). Breviora 539, 1-20 (2014).
3 Lutes, A. A., Baumann, D. P., Neaves, W. B. & Baumann, P. Laboratory synthesis of an independently reproducing vertebrate species. Proc Natl Acad Sci U S A 108, 9910-9915, doi:10.1073/pnas.1102811108 (2011).