Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68588, USA.
Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14850, USA.
G3 (Bethesda). 2022 Aug 25;12(9). doi: 10.1093/g3journal/jkac177.
Crosses between Drosophila simulans females and Drosophila melanogaster males produce viable F1 sons and poorly viable F1 daughters. Unlike most hybrid incompatibilities, this hybrid incompatibility violates Haldane's rule, the observation that incompatibilities preferentially affect the heterogametic sex. Furthermore, it has a different genetic basis than hybrid lethality in the reciprocal cross, with the causal allele in Drosophila melanogaster being a large species-specific block of complex satellite DNA on its X chromosome known as the 359-bp satellite, rather than a protein-coding locus. The causal allele(s) in Drosophila simulans are unknown but likely involve maternally expressed genes or factors since the F1 females die during early embryogenesis. The maternal haploid (mh) gene is an intriguing candidate because it is expressed maternally and its protein product localizes to the 359-bp repeat. We found that this gene has diverged extensively between Drosophila melanogaster and Drosophila simulans. This observation led to the hypothesis that Drosophila melanogaster mh may have coevolved with the 359-bp repeat and that hybrid incompatibility thus results from the absence of a coevolved mh allele in Drosophila simulans. We tested for the functional divergence of mh by creating matched transformants of Drosophila melanogaster and Drosophila simulans orthologs in both Drosophila melanogaster and Drosophila simulans strains. Surprisingly, we find that Drosophila simulans mh fully complements the female sterile phenotype of Drosophila melanogaster mh mutations. Contrary to our hypothesis, we find no evidence that adding a Drosophila melanogaster mh gene to Drosophila simulans increases hybrid viability.
黑腹果蝇模拟种雌性与黑腹果蝇雄性的杂交产生有活力的 F1 雄性后代和活力较差的 F1 雌性后代。与大多数杂种不亲和现象不同，这种杂种不亲和性违反了 Haldane 法则，即不亲和性优先影响异型配子性别。此外，它与正反交杂种致死性具有不同的遗传基础，黑腹果蝇中的因果等位基因是其 X 染色体上一个大型的种特异性复杂卫星 DNA 块，称为 359-bp 卫星，而不是一个蛋白质编码基因座。黑腹果蝇模拟种中的因果等位基因尚不清楚，但可能涉及母体表达的基因或因子，因为 F1 雌性在早期胚胎发生过程中死亡。母体单倍体 (mh) 基因是一个有趣的候选基因，因为它是母体表达的，其蛋白产物定位于 359-bp 重复序列。我们发现，这个基因在黑腹果蝇和黑腹果蝇模拟种之间有广泛的分歧。这一观察结果导致了这样一种假设，即黑腹果蝇 mh 可能与 359-bp 重复序列共同进化，因此杂种不亲和性是由于黑腹果蝇模拟种中缺乏共同进化的 mh 等位基因所致。我们通过在黑腹果蝇和黑腹果蝇模拟种的两个品系中创建黑腹果蝇和黑腹果蝇模拟种同源物的匹配转化体，来检验 mh 的功能分化。令人惊讶的是，我们发现黑腹果蝇模拟种 mh 完全弥补了黑腹果蝇 mh 突变的雌性不育表型。与我们的假设相反，我们没有发现向黑腹果蝇模拟种中添加黑腹果蝇 mh 基因会增加杂种活力的证据。
