Among Anolis lizards, sexual opportunities are typically monopolized by males and female mate choice is low. One way for female anoles to gain back some control in the mating process is through their specialized sperm storage system and selective fertilization. In her talk titled “Females bite back: Sexual conflict and the evolution of venom proteins in the reproductive tract of female anole lizards,” M. Catherine Duryea described her investigation into the genetics of sperm storage in anoles.
First, Duryea asked which genes are expressed in the female reproductive tract after copulation. Duryea extracted tissue from recently mated and virgin female A. carolinensis and generated cDNA libraries. From these libraries, Duryea found that over 160,000 genes were expressed in the reproductive tract, and that 5,153 of these genes were expressed differently in mated versus virgin females. Using a gene ontology analysis, which groups genes by function, Duryea found that many of the genes that showed increased expression in mated females were related to catalytic activity, protein binding, and nucleotide binding. The Anolis genetic response to mating is similar to that reported in Drosophila, suggesting that similar processes may be occurring across distantly related lineages.
Next, Duryea looked for evidence of selection in a subset of the genes identified in the previous experiment. Specifically, she focused on the serine proteases, which are known to be important in sperm storage in Drosophila. Using a BLAST search, Duryea found eight serine protease genes in her A. carolinensis data. She then sequenced the orthologous genes in A. sagrei and compared the sequences to those of A. carolinensis. One serine protease gene showed evidence of positive selection, indicated by a large number of synonymous changes shared between species. This gene displayed striking similarity to a snake venom gene. Snake venom genes have a deep origin in squamates, including in non-venomous lineages; thus, Anolis reproductive serine protease may be derived from a venom serine protease. Compared to Drosophila, in which reproductive serine proteases are derived from digestive enzymes, this would represent a novel origin of reproductive serine proteases.
While these fascinating results are an important first step towards understanding the genetic basis of sperm storage in anoles, much work remains to uncover the exact function of serine protease expression in post-copulatory processes.