It is fascinating that sexually-reproducing organisms employ such diversity of mechanisms to produce males and females, ranging from systems under strict genetic control (GSD) [such as highly dimorphic or undifferentiated sex chromosomes (XY, ZW)], to genetic systems susceptible to some environmental influences [such as haplo-dyploidy, polygenic systems, socially-induced sex reversals], to systems under strict environmental control dependent on biotic or abiotic factors.
TSD appears to be the ancestral state in turtles, and GSD has evolved multiple times in different lineages (Valenzuela and Adams 2011, Sabath et al. 2016), yet little is known about the sex chromosome systems that evolved during the repeated transitions from TSD to GSD. Through a collaborative NSF-funded project (MCB 0815354 - S.V. Edwards CoPI, Harvard University), we tackle the evolution of sex chromosomes and of sex-linked genes, identifying additional sex chromosome systems in turtles and investigating the molecular evolution of the genes they contain.
We use both candidate gene and global approaches address the crucial unanswered question of what molecular factor(s) renders TSD mechanisms thermosensitive. Potential candidates would be genes that express differentially by temperature prior to the onset of the thermosensitive period or TSP (genes organizing or activating the thermosensitive time window rather than those genes acting once the window has opened).
Most of evolutionary biology relies on the assumption that genetic variation (i.e. differences in genomic composition) underlies the diversity of the phenotypes that are exposed to natural selection and allows its evolution. But a great proportion of the phenotypic variation we observe in nature derives from environmental sensitivity of the genome which influences its expression during development (regulatory differences).
Because we are interested in the comparative evolutionary genomics in an ecologically-relevant context, some of the research in my lab investigates questions in population and ecological genetics, life history evolution and conservation biology following previous research (Lance et al. 1992, Valenzuela et al. 1997, Valenzuela 2000, Valenzuela 2001a,b,c, Valenzuela & Janzen 2001, Morjan & Valenzuela 2001, Valenzuela et al. 2003, 2004, Pearse et al. 2006).
Our lab participated in the collaborative project to sequence the painted turtle genome, as part of the Steering Committee for the project, developing transcriptomes of multiple turtle species, carrying out the classic and molecular cytogenetic analyses for its annotation and physical mapping, and conducting analyses to understand the evolution of functional traits such as sex determination