Data from: How fish eggs are preadapted for the evolution of matrotrophy

Teleost fishes evolved livebearing via egg retention 14 times. Matrotrophy has evolved within 12 of those lineages. By contrast, squamate reptiles evolved livebearing over 115 times, but only two to four of those lineages are known to have evolved matrotrophy. Is the discrepancy between these organisms in the probability of this transition caused by differences in their eggs? We show that the eggs of oviparous species in the superorder Atherinomorpha can acquire small organic molecules from their surrounding environment against a concentration gradient via mechanisms of active transport. Uptake rates were inhibited by competing radiolabelled amino acids against unlabelled versions of themselves. Transport was non-specific as uptake rates were similar for l-leucine and its biologically uncommon enantiomer d-leucine. Eggs are also capable of transporting larger microspheres across the membrane, but transport is inhibited at temperatures below 4°C, suggesting active transport occurs via pinocytosis. Conflict theory predicts that the ability of the egg to acquire maternal resources will facilitate the embryo–parent arms race that leads to the evolution of matrotrophy following the transition to livebearing. The shelled eggs of amniotes lack such access to maternal resources when retained in the evolution of viviparity.

真骨鱼类（Teleost fishes）通过卵滞留机制独立演化出胎生（livebearing）习性的次数达14次，其中有12个支系演化出了营养胎生（Matrotrophy）策略。与之形成鲜明对比的是，有鳞目爬行动物（squamate reptiles）演化出胎生习性的次数超过115次，但目前已知其中仅有2至4个支系演化出了营养胎生策略。这两类生物在胎生演化概率上的差异，是否由其卵的结构差异所导致？本研究表明，银汉鱼总目（Atherinomorpha）下卵生物种的卵，可通过主动运输（active transport）机制，逆浓度梯度从周围环境中摄取小分子有机物。当用未标记的氨基酸与放射性标记的同种氨基酸竞争时，卵的摄取速率会受到抑制。该运输过程不具有特异性，因为L-亮氨酸（l-leucine）与其生物学上较为罕见的对映异构体D-亮氨酸（d-leucine）的摄取速率相近。卵还能够将更大尺寸的微球转运穿过细胞膜，但在4℃以下的环境中该转运过程会受到抑制，这表明其主动运输通过胞饮作用（pinocytosis）实现。演化冲突理论（Conflict theory）预测，卵获取母体资源的能力，会推动胚胎与亲代之间的军备竞赛，进而在向胎生的过渡完成后促进营养胎生的演化。羊膜动物（amniotes）的带壳卵在演化出胎生（viviparity）并将卵保留于体内的过程中，无法获得此类母体资源供给途径。