Nabila Saleh-Subaie’s research while affiliated with National Autonomous University of Mexico and other places

A large body of knowledge about life-history traits has arisen from research on viviparous fishes of the family Poeciliidae. Still, the wide variation among species in reproductive strategies provides an excellent opportunity to further explore how life-history traits covary and the causes of covariation patterns. In this study, we provide information on brood size, offspring mass at birth, and total reproductive allotment of six poeciliid species ( Gambusia sexradiata , Poeciliopsis latidens , Poeciliopsis viriosa , Priapella intermedia , Pseudoxiphophorus jonesii , and Xiphophorus hellerii ). Also, we searched for a trade-off between the number of offspring that females produce and the size of each individual offspring. We tested the hypothesis that this trade-off should be stronger in small females because of the space constraints in the reproductive tract that are inherent to a small body size. If this hypothesis were correct, we expected a strong negative relationship between number and size of offspring in small females and a weaker or undetectable relationship between these two life-history traits in larger females. We found evidence of such a size-dependent trade-off in only one species. Small females of Po. latidens that produced relatively large broods experienced the cost of a reduction in the average size of each offspring. In larger females this negative relationship was weaker. Unexpectedly, we found no evidence of this trade-off in the other five poeciliid species and, in contrast, in one species ( Priapella intermedia ) females that produced numerous embryos were also capable of producing relatively large embryos. We discuss potential explanations for the different patterns of covariation (or lack of covariation) between number and size of offspring that we detected in these viviparous species.

The causes and consequences of the evolution of placentotrophy (post-fertilization nutrition of developing embryos of viviparous organisms by means of a maternal placenta) in non-mammalian vertebrates are still not fully understood. In particular, in the fish family Poeciliidae there is an evolutionary link between placentotrophy and superfetation (ability of females to simultaneously bear embryos at distinct developmental stages), with no conclusive evidence for which of these two traits facilitates the evolution of more advanced degrees of the other. Using a robust phylogenetic comparative method based on Ornstein–Uhlenbeck models of adaptive evolution and data from 36 poeciliid species, we detected a clear causality pattern. The evolution of extensive placentotrophy has been facilitated by the preceding evolution of more simultaneous broods. Therefore, placentas became increasingly complex as an adaptive response to evolutionary increases in the degree of superfetation. This finding represents a substantial contribution to our knowledge of the factors that have shaped placental evolution in poeciliid fishes.

The evolution of matrotrophy (post-fertilization maternal provisioning to developing embryos) has been explained through several hypotheses. Trexler and DeAngelis proposed in 2003 a theoretical model that defines the ecological conditions under which matrotrophy would be favored over lecithotrophy (pre-fertilization maternal provisioning). According to this model, matrotrophy offers a selective advantage in environments with abundant and constantly available food, whereas environments with limited and fluctuating food resources should instead promote a lecithotrophic mode of maternal provisioning. This model also proposes that matrotrophy entails the consequence of leaner reproductive females and in turn shorter lifespans. In this study, we examined the Trexler-DeAngelis model using data from 45 populations of five viviparous species from the fish genus Poeciliopsis (family Poeciliidae). We used the matrotrophy index (MI) as a measure of post-fertilization maternal provisioning, and the index of stomach fullness and individual body condition (BC) as proxies for food availability. We also estimated the magnitude of fluctuations in food availability by calculating the temporal variances of these two proxies. Neither abundant nor constantly available food were associated with greater degrees of matrotrophy, which fails to support the predictions of the Trexler-DeAngelis model with respect to the ecological drivers of increased post-fertilization provisioning to embryos. Nonetheless, in all five species we observed that females with greater degrees of matrotrophy had poorer BC compared to females that provided less nutrients to embryos after fertilization. This finding is consistent with one of the expected consequences of advanced matrotrophy according to the Trexler-DeAngelis model, namely, a detriment to the nutritional status of females. Our study provides compelling evidence that gestating females experience a trade-off between post-fertilization provisioning to embryos and self-maintenance, revealing in turn that matrotrophy is a costly reproductive strategy.

In viviparous fishes, females of species that exhibit matrotrophy (post‐fertilization maternal provisioning to developing embryos) and superfetation (ability of females to bear simultaneously multiple broods of embryos at distinct developmental stages) increase less in mass and volume during pregnancy than females of species that lack these traits. Such a reduction in reproductive allocation may provide greater benefits to young and small females for two reasons. First, they could devote energy and resources to both growth and reproduction. Second, they could compensate for the space restrictions that are inherent to a small body size because both superfetation and matrotrophy maximize fecundity for any given ovarian volume. In this study, we test the hypothesis that both matrotrophy and superfetation will decrease over female reproductive lifespan (i.e. as they grow larger). We examined reproductive females from 77 populations of 13 species of the family Poeciliidae. Remarkably, we found support for the matrotrophy prediction in only three populations. As expected, in these populations, small females were more matrotrophic than larger females, which in turn exhibited a predominantly lecithotrophic strategy. In one population, we found the opposite pattern—the degree of matrotrophy actually increased in larger females. With respect to superfetation, none of the populations showed a pattern consistent with our prediction. In fact, in five populations the pattern was opposite to our expectation—larger females produced more simultaneous broods than smaller females. Our findings reveal that the degree of matrotrophy and superfetation can vary throughout the adult lifespans of poeciliid fishes, but such ontogenetic shifts are not common in natural populations.