J. B. Johnson’s research while affiliated with Brigham Young University and other places

In vertebrates, viviparity (livebearing) has evolved independently several times. With the exception of birds, viviparity is found in all vertebrate groups. In bony fishes alone, viviparity has arisen independently at least 13 times, and in cartilaginous fishes at least 9 times. The repeated evolution of viviparity in widely divergent animal lineages has brought a staggering array of morphological, physiological, and behavioral adaptations that facilitate the internal development of embryos. Viviparous fishes have been particularly important model systems for understanding these adaptations, including their function, genetic bases, specialized anatomical features, and the ecological factors that served, and may still serve, as selective agents for the evolution of viviparity.Beyond their role as models to study the proximate and ultimate functions of viviparity in itself, some groups of viviparous fishes, such as the Neotropical families Poeciliidae and Goodeidae, have served as study systems to explore other major genetic, physiological, behavioral, ecological, and evolutionary processes. Through the study of viviparous fishes, a wealth of knowledge has emerged on pre- and post-copulatory sexual selection, complex social behavior, learning and cognition of vertebrates, the origin of unisexual species, genetic variation in natural populations, assembly of freshwater communities, and even the genetic factors involved in the formation of tumors, to name only a few.In this Research Topic, we search for recent, cutting-edge advances in the knowledge of viviparous fishes. In particular, we aim to expand our understanding of the interplay between physiological, reproductive, and behavioral mechanisms, the ecological factors that directly affect the expression of such mechanisms, and the evolutionary forces that have shaped the genetic and phenotypic diversity of all these traits in viviparous fishes. We seek contributions from scientists working across levels of biological organization (genotypes, individuals, populations, and communities) in any group of fishes that include viviparous species. Studies focused on the conservation of threatened taxa, or on the management and control of invasive species, are also welcome.

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.

In viviparous organisms, pregnant females typically experience an increase in body mass and body volume. In this study, the prediction that variation in reproductive traits among populations of viviparous organisms should be related to variation among populations in body shape was tested in the Pacific molly Poecilia butleri, a viviparous fish that inhabits western Mexico and northern Central America. Variation among 10 populations in four reproductive traits was examined: brood size, individual embryo mass, total reproductive allotment and degree of maternal provisioning of nutrients to developing embryos. Variation among these populations in body shape was also examined. Significant variation among populations was observed in both brood size and reproductive allotment but not in embryo mass or degree of maternal provisioning. Significant variation among populations was also observed in body shape. After correcting for female size, however, reproductive traits and body shape were not associated among populations. This suggests that selective pressures acting on reproduction do not necessarily affect morphology and vice versa. Several factors might contribute to this unexpected lack of association between reproductive traits and morphology.