Facultative Matrotrophy in the Livebearing Fish, Gambusia geiseri: Implications for the Evolution of Matrotrophy in the Family Poeciliidae

水蛭科的活鱼表现出各种各样的发育策略。在孔雀鱼中，幼体在卵巢中由储存在大的蛋黄蛋中的能量发育（卵化），而在最小的鳉鱼中，蛋非常小，蛋黄很少，大部分发育和生长的能量通过胎盘样结构从母亲转移到胚胎（专性肌化）。其他物种表现出一种混合策略，在胚胎发育过程中，储存在相对较大的卵子中的能量由母体转移的营养物质补充（补充营养）。一个特定物种所表现出的策略类型被认为反映了它所居住环境的生产力。专性营养物质预计只会出现在高产的环境中，那里母亲的食物丰富且可预测。在生产力较低或难以预测的环境中，脂肪沉积是预期的策略。表现出补充营养的物种可能生活在不可预测的环境中，但如果发生这种情况，它们能够利用多余的食物，并将额外的能量传递给发育中的后代（同时性补充营养）。传统上认为，最大的孕鼠是严格的卵磷脂营养动物，但最近的研究表明，尽管没有明显的胎盘结构，但该物种的营养物质仍可从母体转移到胚胎。本研究建议使用放射性标记的营养物质来量化大孕雄鲸的营养贡献，以解决以下问题：母胚移植表面的性质是什么？它如何限制营养物质转移的数量或类型？在自然条件下，最大突起的gambusia在多大程度上表现出萎缩？在给定种群内和不同种群之间，营养贡献如何随环境条件变化？了解兼性肌萎缩的发生，无论是跨生境的还是在生境内的，可能有助于解释水蛭科的各种发育策略以及这些策略与环境生产力的关系。此外，兼性营养不良策略本身可能反映了对不可预测环境的适应，使具有这种策略的物种能够占据经历自然环境变化的栖息地。因此，这一策略可能为人类引起的环境变化提供一个重要的缓冲。

Livebearing fishes of the Family Poeciliidae exhibit a variety of developmental strategies. In theguppy, young develop in the ovary from the energy stored in large, yolky eggs (lecithotrophy), whereas inthe least killifish, the eggs are very small, with little yolk, and most energy for development and growth istransferred from the mother to the embryo through a placenta-like structure (obligate matrotrophy). Otherspecies exhibit a mixed strategy in which energy stored in relatively large eggs is supplemented bymaternal transfer of nutrients during embryonic development (supplemental matrotrophy). The type ofstrategy exhibited by a given species has been suggested to reflect the productivity of the environmentthat it inhabits. Obligate matrotrophs are predicted to occur only in highly productive environments wherefood for the mother is abundant and predictable. In less productive or less predictable environments,lecithotrophy is the expected strategy. Species that exhibit supplemental matrotrophy may inhabitunpredictable environments but be capable of exploiting excess food if it occurs and passing extra energyon to developing offspring (facultative supplemental matrotrophy).The largespring gambusia has traditionally been considered to be strictly lecithotrophic, butrecent studies have demonstrated mother-to-embryo transfer of nutrients in this species despite theabsence of an obvious placental structure. This study proposes to use radiolabeled nutrients to quantifymatrotrophic contribution in largespring gambusia in order to address the following questions: What isthe nature of the mother-embryo transfer surface and how does it limit the amount or types of nutrientstransferred? To what extent does the largespring gambusia exhibit matrotrophy under natural conditions?How does matrotrophic contribution vary with environmental conditions within a given population andamong different populations? Understanding the occurrence of facultative matrotrophy, both acrosshabitats and temporally within habitats, may help explain the variety of developmental strategies withinthe Family Poeciliidae and the relationship of those strategies to environmental productivity. Furthermore,the strategy of facultative matrotrophy itself may reflect an adaptation to unpredictable environments thatallows species exhibiting this strategy to occupy habitats that undergo natural environmental variation.This strategy, then, may provide an important buffer against human-induced environmental changes.