An essential cell cycle regulation gene causes hybrid inviability in Drosophila

Abstract

Speciation, the process by which new biological species arise, involves the evolution of reproductive barriers, such as hybrid sterility or inviability between populations. However, identifying hybrid incompatibility genes remains a key obstacle in understanding the molecular basis of reproductive isolation. We devised a genomic screen, which identified a cell cycle-regulation gene as the cause of male inviability in hybrids resulting from a cross between Drosophila melanogaster and D. simulans. Ablation of the D. simulans allele of this gene is sufficient to rescue the adult viability of hybrid males. This dominantly acting cell cycle regulator causes mitotic arrest and, thereby, inviability of male hybrid larvae. Our genomic method provides a facile means to accelerate the identification of hybrid incompatibility genes in other model and nonmodel systems.

Figure 1. A genomics screen identifies gfzf^sim as a hybrid inviability gene

a) We mutagenized D. simulans males (new mutations shown in blue) and crossed them to D. melanogaster females. When a D. simulans sperm carrying a mutation at a hybrid incompatibility gene fertilizes a D. melanogaster egg, a viable ‘rescue’ hybrid F1 male is produced. Sequencing the genomes of multiple ‘rescue’ hybrid males identify the causative restorer mutated across these rescue males (shown in red and outlined). b) Single fly genome sequencing of all ‘rescue’ hybrid males allow assignment of new mutations (including large deletions in two of the males) to the D. simulans-derived component of hybrid genomes. c) A single gene, gfzf^sim is mutated across all six ‘rescue’ hybrid F1 males. The X-axis represents the number of genes mutated across all six males, any five males, and so on. The Y-axis represents the number of genes mutated across these males. d) gfzf encodes two alternative transcripts. The larger transcript encodes FLYWCH zinc finger domains along with a GST domain, whereas the shorter transcript encodes only the GST domain.

Figure 2. Knockdown of gfzf^sim rescues cell proliferation defects and restores hybrid male viability

a) No hybrid males are recovered in crosses where the pValium20- gfzf^sim RNAi construct is not expressed (no GAL4 driver, “RNAi OFF”). In crosses between D. simulans males and D. melanogaster females carrying one copy each of pValium20- gfzf^sim and a ubiquitously expressing Actin5C-GAL4 driver, one out of four possible hybrid male progeny inherit both pValium20- gfzf^sim and the Actin5C-GAL4 driver (“RNAi ON”) and produce viable F1 hybrid male progeny. P values were calculated using Fisher’s exact test. b) RNAi knockdown of gfzf^sim by a T80-GAL4 driver, more specific to larval neuroblasts and imaginal discs, successfully restores the viability of F1 male hybrids. c) EdU staining shows the diminutive larval brains and cell proliferation defects in ‘inviable’ hybrid males compared to viable F1 hybrid female larvae. These cell proliferation defects are also partially rescued in hybrid males upon gfzf^sim knockdown.