Embryonic Onset of Sexually Dimorphic Heart Rates in the Viviparous Fish, Gambusia holbrooki

Enhanced mitotic arrest and chromosome resolution for cytogenetic analysis in the eastern mosquitofish, Gambusia holbrooki

Maximising the number of cells arrested at metaphase and their resolution is fundamentally important for molecular cytogenetic investigations, particularly in fish, which typically yield low mitotic index and have highly condensed chromosomes. To overcome these limitations, fish were injected with a mitotic stimulator (the yeast, Saccharomyces cerevisiae) to improve the mitotic index, and the intercalating agent ethidium bromide to produce elongated chromosomes. Specifically, adults were injected with activated yeast and then Colcemid (0.025 µg/µl solution, 10 µl per 1 g of body weight) at 24-96 h post yeast injections, followed by chromosome preparations from multiple tissues. Results showed that gill tissue had the highest number of dividing cells at 72 h post yeast exposure with no significant (p > 0.05) differences between the sexes. Nonetheless, sex-specific differences in the mitotic index were observed in spleen, kidney, and liver, which may be attributed to sex-specific differences in immune responses. For elongation of mitotic chromosomes, individuals (both sexes) were first injected with activated yeast and after 48 h with ethidium bromide (2 or 4 µg/ml) and Colcemid (0.05 µg/µl solution, 10 µl per 1 g of body weight). Following which, animals were sampled at three time points (1, 4 and 8 h) for chromosome preparations. The results show that the optimum elongation of metaphase chromosomes of males and females was achieved by using 2 µg/ml and 4 µg/ml, respectively, for 1 h. Interestingly, the average mitotic chromosome length (μm) of males and females post-ethidium bromide exposure was significantly different (p < 0.05) for both concentrations, except at 1 h exposure for 2 µg/ml EtBr. Such differences can be attributed to overall chromosomal condensation differences between sexes. Regardless, the increased mitotic index and chromosome resolution could benefit cytogenetic studies in other fish species.

The effect of pregnancy on metabolic scaling and population energy demand in the viviparous fish Gambusia affinis

Metabolism is a fundamental attribute of all organisms that influences how species affect and are affected by their natural environment. Differences between sexes in ectothermic species may substantially alter metabolic scaling patterns, particularly in viviparous or live-bearing species where females must support their basal metabolic costs and that of their embryos. Indeed, if pregnancy is associated with marked increases in metabolic demand and alters scaling patterns between sexes, this could in turn interact with natural sex ratio variation in nature to affect population-level energy demand. Here, we aimed to understand how sex and pregnancy influence metabolic scaling and how differences between sexes affect energy demand in Gambusia affinis (Western mosquitofish). Using the same method, we measured routine metabolic rate in the field on reproductively active fish and in the laboratory on virgin fish. Our data suggest that changes in energy expenditure related to pregnancy may lead to steeper scaling coefficients in females (b = 0.750) compared to males (b = 0.595). In contrast, virgin females and males had similar scaling coefficients, suggesting negligible sex differences in metabolic costs in reproductively inactive fish. Further, our data suggest that incorporating sex differences in allometric scaling may alter population-level energy demand by as much as 20-28%, with the most pronounced changes apparent in male-biased populations due to the lower scaling coefficient of males. Overall, our data suggest that differences in energy investment in reproduction between sexes driven by pregnancy may alter allometric scaling and population-level energy demand.

Sex-Specific Life History Affected by Stocking in Juvenile Brown Trout

Salmonids are a socioeconomically and ecologically important group of fish that are often managed by stocking. Little is known about potential sex-specific effects of stocking, but recent studies found that the sexes differ in their stress tolerances already at late embryonic stage, i.e., before hatchery-born larvae are released into the wild and long before morphological gonad formation. It has also been speculated that sex-specific life histories can affect juvenile growth and mortality, and that a resulting sex-biassed demography can reduce population growth. Here we test whether juvenile brown trout (Salmo trutta) show sex-specific life histories and whether such sex effects differ in hatchery- and wild-born fish. We modified a genetic sexing protocol to reduce false assignment rates and used it to study the timing of sex differentiation in a laboratory setting, and in a large-scale field experiment to study growth and mortality of hatchery- and wild-born fish in different environments. We found no sex-specific mortality in any of the environments we studied. However, females started sex differentiation earlier than males, and while growth rates were similar in the laboratory, they differed significantly in the field depending on location and origin of fish. Overall, hatchery-born males grew larger than hatchery-born females while wild-born fish showed the reverse pattern. Whether males or females grew larger was location-specific. We conclude that juvenile brown trout show sex-specific growth that is affected by stocking and by other environmental factors that remain to be identified.

Stages of embryonic development in the live-bearing fish, Gambusia holbrooki

BACKGROUND

Divergent morphology and placentation of Poeciliids make them suitable model for investigating how evolutionary selection has altered and conserved the developmental mechanisms. However, there is limited description of their embryonic staging, despite representing a key evolutionary node that shares developmental strategy with placental vertebrates. Here, we describe the embryonic developmental stages of Gambusia holbrooki from zygote to parturition using freshly harvested embryos.

RESULTS

We defined 40 embryonic stages using a numbered (stages 0-39; zygote to parturition, respectively) and named (grouped into seven periods, ie, zygote, cleavage, blastula, gastrula, segmentation, pharyngula, and parturition) staging system. Two sets of quantitative (ie, egg diameter, embryonic total length, otic vesicle closure index, heart rates, the number of caudal fin rays and elements) and qualitative (ie, three-dimensional analysis of images and key morphological criteria) data were acquired and used in combination to describe each stage. All 40 stages are separated by well-defined morphological traits, revealing developmental novelties that are influenced by narrow perivitelline space, placentation, internal gestation, and sex differentiation.

CONCLUSIONS

The principal diagnostic features described are quick, reliable, and easy to apply. This system will benefit researchers investigating molecular ontogeny, particularly sexual differentiation mechanisms in G. holbrooki.