Abstract
BackgroundThe East African riverine cichlid species Astatotilapia burtoni serves as an important laboratory model for sexually dimorphic physiology and behavior, and also serves as an outgroup species for the explosive adaptive radiations of cichlid species in Lake Malawi and Lake Victoria. An astounding diversity of genetic sex determination systems have been revealed within the adaptive radiation of East African cichlids thus far, including polygenic sex determination systems involving the epistatic interaction of multiple, independently segregating sex determination alleles. However, sex determination has remained unmapped in A. burtoni. Here we present mapping results supporting the presence of multiple, novel sex determination alleles, and thus the presence of polygenic sex determination in A. burtoni.ResultsUsing mapping in small families in conjunction with restriction-site associated DNA sequencing strategies, we identify associations with sex at loci on linkage group 13 and linkage group 5–14. Inheritance patterns support an XY sex determination system on linkage group 5–14 (a chromosome fusion relative to other cichlids studied), and an XYW system on linkage group 13, and these associations are replicated in multiple families. Additionally, combining our genetic data with comparative genomic analysis identifies another fusion that is unassociated with sex, with linkage group 8–24 and linkage group 16–21 fused in A. burtoni relative to other East African cichlid species.ConclusionsWe identify genetic signals supporting the presence of three previously unidentified sex determination alleles at two loci in the species A. burtoni, strongly supporting the presence of polygenic sex determination system in the species. These results provide a foundation for future mapping of multiple sex determination genes and their interactions. A better understanding of sex determination in A. burtoni provides important context for their use in behavioral studies, as well as studies of the evolution of genetic sex determination and sexual conflicts in East African cichlids.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-3177-1) contains supplementary material, which is available to authorized users.
Highlights
The East African riverine cichlid species Astatotilapia burtoni serves as an important laboratory model for sexually dimorphic physiology and behavior, and serves as an outgroup species for the explosive adaptive radiations of cichlid species in Lake Malawi and Lake Victoria
A. burtoni scaffold assignment was consistent by linkage group for both species, but regular inconsistencies in scaffold order indicate a moderate level of intrachromosomal rearrangement between O. niloticus and M. zebra
Markers aligning to two tilapia linkage groups, LG8-24 and LG16-21, were linked in our map output. (The nomenclature for both LG8-24 and LG16-21 in tilapia does not indicate fused chromosomes, but is instead a historical artifact: the standard O. niloticus linkage map originally had 24 linkage groups [73], but later improvement of the genetic map identified 22 linkage groups [42], collapsing the former LG8 and LG24 into a single linkage group, as well as LG16 and LG21.) We find continuous haplotypes spanning LG8-24 and LG16-21, with significant linkage between markers aligning to the two linkage groups (LOD = 10.3, Additional file 4), supporting the fusion of the two associated chromosomes in A. burtoni relative to other cichlids
Summary
The East African riverine cichlid species Astatotilapia burtoni serves as an important laboratory model for sexually dimorphic physiology and behavior, and serves as an outgroup species for the explosive adaptive radiations of cichlid species in Lake Malawi and Lake Victoria. Sex determination is absolutely fundamental to the fitness of sexually reproducing organisms, yet the “switches” modulating female versus male development are incredibly diverse [1]. This diversity arises from surprisingly numerous transitions in sex determination mechanisms across taxa, yet little is know about the evolutionary and developmental forces underlying these transitions. Given the current catalog of identified sex determination mechanisms, fish are Roberts et al BMC Genomics (2016) 17:835 allele is associated with female development (ZW females, ZZ males). As sex determination alleles are cataloged, surprising overlap suggests repeated modification of a few pathways across broad taxa to modulate sexual development. Recent identification of sex determiners such as gsdf and sexually dimorphic on the Y-chromosome (sdY) within fish make it clear that sex determination mechanisms are not limited to the above pathways [15, 16]
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