(Evolutionary Genetics and Genomics)

Time：2023. 02. 17 Fri. 15:00

Venue: Auditorium, 1st Floor, Interdisciplinary Research Building

Speaker：Dr. Ching-Ho Chang
                     Postdoctoral Fellow
                     Basic Science Division, Fred Hutchinson Cancer center, USA

Title：Genetic Conflicts Drive Rapid Evolution of Protamines in Drosophila

Host：Dr. Isheng Jason Tsai

Abstract
      My research program aims to understand how genetic conflicts can shape rapid evolution of even essential chromosomal proteins like protamines. Many animal species employ protamines to package sperm genomes tightly. Despite their essential requirements for male fertility in many animal species, protamines vary across animal lineages and evolve rapidly in mammals. Their rapid evolution is unexplained and could result from relentless competition between sperm for fertilization success or genetic conflicts with selfish genetic elements in the male germline. I used a phylogenomic approach to investigate protamine diversification in Drosophila species. I found that most protamine genes in Drosophila melanogaster evolve under positive selection. Unexpectedly, evolutionarily young protamine genes are more likely to be critical for fertility than ancient, conserved protamine genes. For example, I found that ancient CG30056 is dispensable for male fertility despite being one of three protamine genes universally retained in Drosophila species, whereas both ‘young’ protamine genes Prtl99C and Mst77F are essential for fertility. I found 19 independent protamine gene amplification events that occurred preferentially on sex chromosomes. Conversely, the montium group of Drosophila species lost otherwise-conserved protamine genes, coincident with an X-Y chromosomal fusion. My observations lead to the hypothesis that autosomal protamine genes suppress meiotic drive, whereas sex-chromosomal protamine expansions lead to meiotic drive. X-Y fusions in the montium group render autosomal protamines dispensable by making X-versus-Y meiotic drive obsolete or costly, resulting in their loss. Thus, genetic conflicts between sex chromosomes may drive protamines rapid evolution during spermatogenesis in Drosophila species. Using gene-swap analyses I am currently testing this hypothesis. I found that replacing fertility-essential Mst77F-mel with a distantly related species Mst77-ana leads to sterility, whereas replacement with closely related species Mst77F-sim or Mst77F-yak leads to recovery of fertility but with male-biased progeny. My findings support the hypothesis that evolution of Mst77F is driven to suppress X-versus-Y sperm killing. Thus, genetic conflicts explain the paradoxically rapid evolution of fertility-essential protamine genes.

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