Our understanding of the evolution of complex biological traits is greatly advanced by examining taxa with intermediate phenotypes. The transition from oviparity (egg-laying) to viviparity (live-bearing) has occurred independently in many animal lineages, but there are few phenotypic intermediates. The lizard Saiphos equalis exhibits bimodal reproduction, with some viviparous populations, and other oviparous populations with long egg-retention, a rare trait where most of embryonic development occurs inside the mother prior to late ovipositioning. We posit that oviparous S. equalis represent an intermediate form between "true" oviparity and viviparity. We used transcriptomics to compare uterine gene expression in these two phenotypes, and provide a molecular model for the genetic control and evolution of reproductive mode. Many genes are differentially expressed throughout the reproductive cycle of both phenotypes, which have clearly different gene expression profiles overall. The differentially expressed genes within oviparous and viviparous individuals have broadly similar biological functions putatively important for sustaining embryos, including uterine remodelling, respiratory gas and water exchange, and immune regulation. These functional similarities indicate either that long egg-retention is an exaptation for viviparity, or might reflect parallel evolution of similar gravidity-related changes in gene expression in long egg-retention oviparity. In contrast, gene expression changes across the reproductive cycle of long egg-retaining oviparous S. equalis are dramatically different from those of "true" oviparous skinks (such as Lampropholis guichenoti), supporting our assertion that oviparous S. equalis exhibit an intermediate phenotype between "true" oviparity and viviparity.

中文翻译：

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进化创新的出现：与卵胎和卵胎之间过渡相关的基因表达差异。

通过检查具有中间表型的分类单元，大大提高了我们对复杂生物学特征进化的理解。从卵生（产卵）到胎生（活产）的过渡在许多动物谱系中独立发生，但很少有表型中间体。蜥蜴Saiphos equalis表现出双峰繁殖，有一些胎生种群，而其他卵生种群具有长卵期，这是一种罕见的性状，大多数胚胎发育都在晚排卵前发生在母亲体内。我们认为，卵生的S. equiis代表“真实”卵生和卵生之间的中间形式。我们使用转录组学技术比较了这两种表型中的子宫基因表达，并提供了遗传控制和生殖方式进化的分子模型。在两种表型的整个繁殖周期中，许多基因差异表达，它们的整体基因表达谱明显不同。卵生和卵生个体中差异表达的基因具有广泛相似的生物学功能，可能对维持胚胎具有重要作用，包括子宫重塑，呼吸气体和水交换以及免疫调节。这些功能上的相似性要么表明长期保留卵是卵胎分离，要么可能反映了长期保留卵卵子基因表达中相似的引力相关变化的平行演变。相反，在长卵期卵生卵小链球菌的整个生殖周期中，基因表达的变化与“真”卵的变异显着不同。