Reproductive isolation caused by azoospermia in sterile male hybrids of Drosophila

Abstract

Recently diverged populations in the early stages of speciation offer an opportunity to understand mechanisms of isolation and their relative contributions. Drosophila willistoni is a tropical species with broad distribution from Argentina to the southern United States, including the Caribbean islands. A postzygotic barrier between northern populations (North America, Central America, and the northern Caribbean islands) and southern populations (South American and the southern Caribbean islands) has been recently documented and used to propose the existence of two different subspecies. Here, we identify premating isolation between populations regardless of their subspecies status. We find no evidence of postmating prezygotic isolation and proceeded to characterize hybrid male sterility between the subspecies. Sterile male hybrids transfer an ejaculate that is devoid of sperm but causes elongation and expansion of the female uterus. In sterile male hybrids, bulging of the seminal vesicle appears to impede the movement of the sperm toward the sperm pump, where sperm normally mixes with accessory gland products. Our results highlight a unique form of hybrid male sterility in Drosophila that is driven by a mechanical impediment to transfer sperm rather than by an abnormality of the sperm itself. Interestingly, this form of sterility is reminiscent of a form of infertility (azoospermia) that is caused by lack of sperm in the semen due to blockages that impede the sperm from reaching the ejaculate.

Gene expression differentiation in the reproductive tissues of Drosophila willistoni subspecies and their hybrids

“…Instead, we found several GO terms related to different aspects of cell adhesion, which is important for the proper assembly of the muscle sheaths in the D. melanogaster male reproductive system (Kuckwa et al, 2016; Susic‐Jung et al, 2012). The low proportion of misregulated accessory gland genes in hybrid males aligns with the absence of differences in seminal fluid composition of the sterile males, as these are able to induce the typical changes in uterus morphology (Davis et al, 2020). Lastly, in ovary, chorion‐containing eggshell formation genes were over‐represented among those showing transgressive expression, possibly reflecting a fast regulatory divergence between subspecies, similarly to their evolutionary rate at the protein sequence level (Jagadeeshan & Singh, 2007).…”

“…The directionality of the arrow denotes whether the significant association represents enrichment or depletion. (Davis et al, 2020;Mardiros et al, 2016). This unidirectional hybrid male sterility plus a negligible cell-type tissue composition differentiation in the sterile hybrid make this subspecies pair well poised to address how regulation in gene expression evolves during the early stages of speciation across tissues, resulting in hybrid dysfunction.…”

“…We have comprehensively compared the transcriptome of two D. willistoni subspecies, D. w. willistoni and D. w. winge , and the hybrids resulting from the cross between them that yields sterile males (Davis et al, 2020; Mardiros et al, 2016). This unidirectional hybrid male sterility plus a negligible cell‐type tissue composition differentiation in the sterile hybrid make this subspecies pair well poised to address how regulation in gene expression evolves during the early stages of speciation across tissues, resulting in hybrid dysfunction.…”

“…The first two subspecies are not only indistinguishable at the morphological level, including fast evolving traits such as the male genitalia (Civetta & Gaudreau, 2015), but also in terms of sequence differentiation as revealed by the gene mtCOI (Mardiros et al, 2016). Although no fixed premating isolation has been found between these subspecies (Davis et al, 2020), they do exhibit unidirectional hybrid male sterility (Mardiros et al, 2016). Notably, and unlike other sterile hybrids between recently diverged Drosophila lineages that show sperm defects and reduced sperm motility (Gomes & Civetta, 2014; Kulathinal & Singh, 1998; Moehring et al, 2006), these sterile hybrid males produce typical numbers of motile sperm but cannot transfer them during copulation (Civetta & Gaudreau, 2015; Gomes & Civetta, 2014).…”

“…Canada, Grant/Award Number: RGPIN-2017-04599; University of California Institute for Mexico and the United States Handling Editor: Christian Schlötterer female(Davis et al, 2020). Collectively, the features of the D. w. willistoni and D. w. winge subspecies are largely suggestive of a very early stage in their process of consolidation as distinct species, offering an excellent opportunity to detect misregulation in their hybrids while discerning the role of different regulatory elements on expression divergence under minimal biases introduced by dissimilar cell type composition.…”