Table_5_Asymmetric Phenotypes of Sterile Hybrid Males From Reciprocal Crosses Between Species of the Anopheles gambiae Complex.XLSX

Haldane’s rule of speciation states that sterility or inviability affects the heterogametic sex of inter-species hybrids. Darwin’s corollary to Haldane’s rule implies that there are asymmetric phenotypes in inter-species hybrids from reciprocal crosses. Studying the phenotypes of F1 hybrids among closely related species of malaria mosquitoes can assist researchers in identifying the genetic factors and molecular mechanisms of speciation. To characterize phenotypes of sterile hybrid males in the Anopheles gambiae complex, we performed crosses between laboratory strains of An. merus and either An. gambiae or An. coluzzii. The reproductive tracts had normal external morphology in hybrid males from crosses between female An. merus and male An. gambiae or An. coluzzii. Despite being sterile, these males could copulate with females for a normal period of time and could transfer a mating plug to induce female oviposition and monogamy. In contrast, the entire reproductive tracts in hybrid males from crosses between female An. gambiae or An. coluzzii and male An. merus were severely underdeveloped. These males had atrophic testes and reduced somatic organs of the reproductive system including male accessary glands and ejaculatory duct. In addition, hybrid males with underdeveloped reproductive tracts displayed a shorter copulation time with females and failed to induce female oviposition and monogamy due to their inability to form and transfer a plug to females during mating. The asymmetry of the phenotypes associated with hybrid male sterility suggests that different genetic factors and molecular mechanisms are responsible for reproductive isolation in reciprocal crosses among species of the An. gambiae complex.

霍尔丹物种形成法则（Haldane’s rule of speciation）提出，物种间杂种的不育或致死会影响异配性别（heterogametic sex）。达尔文对霍尔丹法则的推论表明，正反交（reciprocal crosses）产生的物种间杂种会呈现不对称表型。研究疟疾蚊虫近缘物种间的F1杂种（F1 hybrid）表型，可帮助研究者揭示物种形成的遗传因素与分子机制。为了表征冈比亚按蚊复合体（Anopheles gambiae complex）中不育杂种雄蚊的表型，我们开展了迷走按蚊（An. merus）实验室品系与冈比亚按蚊（An. gambiae）或科兹按蚊（An. coluzzii）实验室品系的杂交实验。当母本为迷走按蚊、父本为冈比亚按蚊或科兹按蚊时，所得杂种雄蚊的生殖系统外部形态正常。尽管这类雄蚊不育，但它们可与雌蚊进行正常时长的交配，并能转移交配栓以诱导雌蚊产卵与单配制行为。与之相反，当母本为冈比亚按蚊或科兹按蚊、父本为迷走按蚊时，所得杂种雄蚊的整个生殖系统均出现严重发育不全。这类雄蚊的睾丸发生萎缩，生殖系统的体细胞器官（包括雄性副性腺与射精管）均出现退化。此外，生殖系统发育不全的杂种雄蚊，与雌蚊的交配时长更短，且无法诱导雌蚊产卵与单配制行为，究其原因是它们无法在交配过程中形成并向雌蚊转移交配栓。这种与杂种雄性不育相关的表型不对称性表明，冈比亚按蚊复合体各物种间的正反交生殖隔离，由不同的遗传因素与分子机制所介导。