This project will illuminate the evolution of dosage compensation (DC) by contrasting turtle species with XX/XY, ZZ/ZW and temperature-dependent (TSD) gender determination. In species like humans, XX/XY females have two copies of genes in the X chromosome and males one because females carry two Xs and males one X and one Y. An imbalance in gene dosage, such the one caused by the difference in the number of X or Z chromosomes between genders, may reduce fitness or even cause death, without a mechanism to re-establish a balance. Dosage compensation mechanisms (DCM) equalize the activity of genes located in the X or Z (X-linked or Z-linked) between chromosomes, and between males and females. DC is important for normal development, and has consequences for the evolution of genomes and speciation. Yet, DCM diversity and evolution remain unclear. Turtles are ideal to study DCM evolution because XX/XY and ZZ/ZW chromosomes have evolved independently multiple times, and there are increasing turtle genomic resources. To address this puzzle, gene expression is being compared in turtles with contrasting mechanisms of gender determination. The results will advance our understanding of the diversity and evolution of DMC by illuminating whether there are general rules to DCM across organisms or whether unique solutions evolved independently in each lineage. This project involves undergraduate discovery-based learning by recruiting biology undergraduates, and broadens the participation of underrepresented groups in biology. Undergraduates will be trained in a variety of biological fields, participate in reflection sessions and present their results at scientific meetings. Results will be disseminated via presentations, publications, and a bilingual web-site.
When XX/XY or ZZ/ZW chromosomes evolve in diploid species, genes are lost from the Y or W (or lose their function) as they degenerate over time due to their lower recombination. This loss means that XY-males carry a single functional copy of X-genes while XX-females carry two copies (likewise, ZW females have one copy of Z-genes and ZZ-males have two copies). Because X- and Z-linked genes are part of genetic networks, the inequality of gene copy number causes an imbalance in transcription which may produce sub-optimal or lethal phenotypes, unless a mechanism existed to re-establish a balance. In the absence of such mechanism, transcription will differ (a) between genes in these chromosomes and autosomes in XY males (or ZW females), and (b) between males and females for X-genes (or Z-genes). Dosage compensation (DC) is a system to equalize the activity of X- or Z-linked genes to counterbalance the deleterious effects of differential gene copy number. DC re-establishes a balance between these genes and autosomal genes ("chromosome-to-autosome" compensation), and between the genders ("male-to-female" compensation). Mechanisms may balance entire chromosomes (global compensation), other some genes only (local compensation). This project tests the hypothesis that global DC (i.e. chromosome-wide) does not evolve inevitably with the evolution of chromosomes, but instead that DC tends to be local when present (i.e. affecting some genes only), and independent of the size, evolutionary age, and gene content of the chromosomes. Accordingly, this project it is sequencing microdissected turtle chromosomes, and examining them by arrayCGH, and assessing the expression of these genes via transcriptomics.
