Testis- and ovary-expressedpolotranscripts and gene duplications affect male fertility when expressed in the germline

Abstract

AbstractPolo-like kinases (Plks) are essential for spindle attachment to the kinetochore during prophase and the subsequent dissociation after anaphase in both mitosis and meiosis. There are structural differences in the spindle apparatus between mitosis, male meiosis, and female meiosis. It is therefore possible that alleles of Plk genes could improve kinetochore attachment or dissociation in spermatogenesis or oogenesis, but not both. These opposing effects could result in sexually antagonistic selection at Plk loci. In addition, Plk genes have been independently duplicated in many different evolutionary lineages within animals. This raises the possibility that Plk gene duplication may resolve sexual conflicts over mitotic and meiotic functions. We investigated this hypothesis by comparing the evolution, gene expression, and functional effects of the single Plk gene inDrosophila melanogaster(polo) and the duplicated Plks inDrosophila pseudoobscura(Dpse-poloandDpse-polo-dup1). We found that the protein-coding sequence ofDpse-polo-dup1is evolving significantly faster than a canonicalpologene across all functional domains, yet the essential structure of encoded protein appears to be retained.Dpse-polo-dup1is expressed primarily in testis, while otherpologenes have broader expression profiles. Furthermore, over or ectopic expression ofpoloorDpse-poloin theD. melanogastermale germline results in greater male infertility than ectopic expression ofDpse-polo-dup1. Lastly, ectopic expression ofDpse-poloor an ovary-derived transcript ofpoloin the male germline causes males to sire female-biased broods. However, there is no sex-bias in the progeny whenDpse-polo-dup1is ectopically expressed or a testis-derived transcript ofpolois overexpressed in theD. melanogastermale germline. Our results therefore suggest thatDpse-polo-dup1may have experienced positive selection to improve its regulation of the male meiotic spindle, resolving sexual conflict over meiotic Plk functions. Alternatively,Dpse-polo-dup1may encode a hypomorphic Plk that has reduced deleterious effects when overexpressed in the male germline. Similarly, testis transcripts ofD. melanogaster polomay be optimized for regulating the male meiotic spindle, and we provide evidence that the untranslated regions of thepolotranscript may be involved in sex-specific germline functions.