A Burst of Genetic Innovation in Drosophila Actin-Related Proteins for Testis-Specific Function

Abstract

Many cytoskeletal proteins perform fundamental biological processes and are evolutionarily ancient. For example, the superfamily of actin-related proteins (Arps) specialized early in eukaryotic evolution for diverse cellular roles in the cytoplasm and the nucleus. Despite its strict conservation across eukaryotes, we find that the Arp superfamily has undergone dramatic lineage-specific diversification in Drosophila. Our phylogenomic analyses reveal four independent Arp gene duplications that occurred in the common ancestor of the obscura group of Drosophila and have been mostly preserved in this lineage. All four obscura-specific Arp paralogs are predominantly expressed in the male germline and have evolved under positive selection. We focus our analyses on the divergent Arp2D paralog, which arose via a retroduplication event from Arp2, a component of the Arp2/3 complex that polymerizes branched actin networks. Computational modeling analyses suggest that Arp2D can replace Arp2 in the Arp2/3 complex and bind actin monomers. Together with the signature of positive selection, our findings suggest that Arp2D may augment Arp2’s functions in the male germline. Indeed, we find that Arp2D is expressed during and following male meiosis, where it localizes to distinct locations such as actin cones—specialized cytoskeletal structures that separate bundled spermatids into individual mature sperm. We hypothesize that this unprecedented burst of genetic innovation in cytoskeletal proteins may have been driven by the evolution of sperm heteromorphism in the obscura group of Drosophila.

A rapidly evolving actin mediates fertility and developmental tradeoffs inDrosophila

“…We hypothesize that the requirement of these specialized actin processes may have led to the independent origin and retention of many non-canonical Arps throughout animal evolution. Indeed, we find that another non-canonical Arp that arose via gene duplication from canonical Arp2 in the D. pseudoobscura lineage, independently specialized to localize to actin cones 18 . We speculate that their role in reproduction, as a class, may have led to their accelerated rate of evolution due to strong selective pressures from sperm competition and sexual selection [60][61][62] .…”

“…However, phylogenomic surveys reveal that 'non-canonical' Arps are not as rare as previously believed. Recently, we described a 14-million-year-old Drosophila clade that independently acquired four non-canonical Arp genes, which are all expressed solely in the male germline 18 . Mammals also encode at least seven non-canonical Arps predominantly expressed in the testis [19][20][21][22][23] , at least some of which localize to actin structures in sperm development 21,22 .…”

An actin-related protein that is most highly expressed in Drosophila testes is critical for embryonic development

“…The specialized requirement of these actin processes may have led to the independent origin and retention of many non-canonical Arps throughout animal evolution. Indeed, we find that another non-canonical Arp, which independently arose via gene duplication from canonical Arp2 in the D. pseudoobscura lineage, also specialized to localize to actin cones ( Schroeder et al, 2020 ). Their role in reproduction may have also led to their accelerated rate of evolution due to strong selective pressures from sperm competition and sexual selection ( Kleene, 2005 ; Swanson and Vacquier, 2002 ; Panhuis et al, 2006 ).…”

“…Many non-canonical Arps show testes-enriched patterns of expression ( Schroeder et al, 2020 ; Heid et al, 2002 ; Hara et al, 2008 ; Fu et al, 2012 ; Harata et al, 2001 ). It is not unexpected that novel Arps might specialize for spermatogenesis, which requires several novel cytoskeletal functions and complex actin structures.…”

“…However, phylogenomic surveys reveal that ‘non-canonical’ Arps are not as rare as previously believed. Recently, we described a 14-million-year-old Drosophila clade that independently acquired four non-canonical Arp genes that are all expressed primarily in the male germline ( Schroeder et al, 2020 ). Mammals also encode at least seven non-canonical Arps that are predominantly expressed in the testis ( Heid et al, 2002 ; Tanaka et al, 2003 ; Hara et al, 2008 ; Boëda et al, 2011 ; Fu et al, 2012 ), at least some of which localize to actin structures in sperm development ( Hara et al, 2008 ; Boëda et al, 2011 ).…”