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

Gonad Ontogeny and Sex Differentiation in a Poeciliid, Gambusia holbrooki: Transition from a Bi- to a Mono-Lobed Organ

Despite their uniqueness, the ontogeny and differentiation of the single-lobed gonads in the poeciliids are very poorly understood. To address this, we employed both cellular and molecular approaches to systematically map the development of the testes and ovary in Gambusia holbrooki from pre-parturition to adulthood, encompassing well over 19 developmental stages. The results show that putative gonads form prior to the completion of somitogenesis in this species, a comparatively early occurrence among teleosts. Remarkably, the species recapitulates the typical bi-lobed origin of the gonads during early development that later undergoes steric metamorphosis to form a single-lobed organ. Thereafter, the germ cells undergo mitotic proliferation in a sex-dependent manner before the acquisition of the sexual phenotype. The differentiation of the ovary preceded that of the testes, which occurred before parturition, where the genetic females developed meiotic primary oocytes stage I, indicating ovarian differentiation. However, genetic males showed gonial stem cells in nests with slow mitotic proliferation at the same developmental stage. Indeed, the first signs of male differentiation were obvious only post-parturition. The expression pattern of the gonadosoma markers foxl2, cyp19a1a, amh and dmrt1 in pre- and post-natal developmental stages were consistent with morphological changes in early gonad; they were activated during embryogenesis, followed by the onset of gonad formation, and a sex-dimorphic expression pattern concurrent with sex differentiation of the ovary (foxl2, cyp19a1a) and testes (amh and dmrt1). In conclusion, this study documents for the first time the underlying events of gonad formation in G. holbrooki and shows that this occurs relatively earlier than those previously described for ovi- and viviparous fish species, which may contribute to its reproductive and invasive prowess.

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.

Primordial Germ Cell Development in the Poeciliid, Gambusia holbrooki, Reveals Shared Features Between Lecithotrophs and Matrotrophs

Metazoans exhibit two modes of primordial germ cell (PGC) specification that are interspersed across taxa. However, the evolutionary link between the two modes and the reproductive strategies of lecithotrophy and matrotrophy is poorly understood. As a first step to understand this, the spatio-temporal expression of teleostean germ plasm markers was investigated in Gambusia holbrooki, a poecilid with shared lecitho- and matrotrophy. A group of germ plasm components was detected in the ovum suggesting maternal inheritance mode of PGC specification. However, the strictly zygotic activation of dnd-β and nanos1 occurred relatively early, reminiscent of models with induction mode (e.g., mice). The PGC clustering, migration and colonisation patterns of G. holbrooki resembled those of zebrafish, medaka and mice at blastula, gastrula and somitogenesis, respectively-recapitulating features of advancing evolutionary nodes with progressive developmental stages. Moreover, the expression domains of PGC markers in G. holbrooki were either specific to teleost (vasa expression in developing PGCs), murine models (dnd spliced variants) or shared between the two taxa (germline and somatic expression of piwi and nanos1). Collectively, the results suggest that the reproductive developmental adaptations may reflect a transition from lecithotrophy to matrotrophy.