Actin gone wild

“There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

Charles Darwin, The Origin of Species

Actin is an ancient and abundant protein with well-established roles in many fundamental processes ranging from cellular architecture to signaling, transport, and migration. The evolution of the canonical actin protein pre-dates eukaryotic divergence. This stringent conservation of actin speaks to how fundamental actin is to eukaryotic biology.

Most eukaryotic cells also contain several actin-related proteins (ARPs) that are, themselves, conserved between organisms as divergent as yeast and mammals. They all share a conserved sequence with the canonical actin fold. Dr. Harmit Malik and his laboratory (Basic Sciences Division) recently uncovered an unusual burst of genetic diversification in ARPs among several Drosophila species. Dr. Courtney Schroeder, a postdoc in the Malic lab, led the study. Their findings are published in a recent issue of Molecular Biology and Evolution.

Dr. Schroeder explained their findings: “Arps are considered to be evolutionarily ancient, with the superfamily of Arps originating before the last common ancestor of eukaryotes. They tend to be under strong sequence conservation. However, our study discovered not one but four Arp gene duplicates in a single clade of Drosophila species. And these Arps, unlike canonical Arps, are rapidly diversifying in sequence, or changing at a rate faster than expected for such a short window of evolutionary time.”

Previously published phylogenetic analyses of Arp family evolution highlighted distinct and well-defined subfamilies of Arps. Proteins within each Arp subfamily were found to be highly similar indicating the strong selective pressure for conservation of both sequence and function for each Arp following initial diversification billions of years ago. This well-controlled diversification of Arp proteins facilitated the identification of rare variants that became known as “orphan” Arps. One such example are the seven testis-specific Arps found in mammals with no equivalent outside mammals and little is known about them.

Dr. Schroeder and her colleagues wanted to investigate orphan Arps in Drosophila hoping to shed the light on novel molecular roles and evolution of Arps. They took advantage of recent sequencing of the genomes of different species of Drosophila to analyze the evolution of Arp proteins by performing phylogenetic analysis. Their analysis uncovered a burst of lineage-specific duplications in the obscura group of Drosophila. Since Arps appear to be stringently conserved among eukaryotic species, it was a surprise to them to see this burst of Arp diversification. Why did this group of Drosophila Arps evolve so rapidly? 

“This genetic innovation appears to be specializing for roles in the male germline. Unlike canonical Arps, which are expressed ubiquitously, the Arp gene duplicates are testis-specific in expression” explained Dr. Schroeder. The researchers focused their analysis on the divergent paralog Arp2D, a product of a retroduplication event from Arp2, a component of the Arp2/3 complex that polymerizes branched actin networks. Their findings suggested that Arp2D, which they found to be testis-specific, may work to augment Arp2’s functions in the male germline.

Commenting on some of the challenges in their study, she said: “Since the testis-specific Arps were discovered in a non-traditional model organism, it is more difficult to generate transgenic flies. We were lucky to obtain a D. pseudoobscura transgenic encoding a fluorescently tagged Arp2D, which allowed us to visualize where this Arp localizes. Since we found Arp2D-GFP localizes to actin, we hypothesize that male gametic Arps play actin regulatory roles.” Indeed, 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.

An answer to why this group of Drosophila Arps evolves so rapidly maybe found in the evolutionary process driving sperm heteromorphism in the obscura group of Drosophila. The obscura clade is odd in that it has long and short sperm. The short sperm, or “kamikaze” sperm, are thought to protect the long sperm, which is the only sperm type that can fertilize the female egg. It raises however many questions for future investigation. “Not only do testis-specific Arps exist in flies, but there are even testis-specific Arps found in humans. And we have no idea what the function of a testis-specific Arp is in flies or humans,” said Dr. Schroeder. “Why are there recurrent gene duplications of Arps across phyla, and how are these duplicates diversifying for roles in the male germline?” she pondered. Future research may help answer these questions. 

Schroeder CM, Valenzuela JR, Natividad IM, Hocky GM, Malik HS. 2019. A burst of genetic innovation in Drosophila actin-related proteins for testis-specific function. Molecular Biology and Evolution.

This work was funded by the Jane Coffin Childs Memorial Fund, the National Institute of Health and the Howard Hughes Medical Institute.