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

Abstract

In fish, little is known about sex-specific differences in physiology and performance of the heart and whether these differences manifest during development. Here for the first time, the sex-specific heart rates during embryogenesis of Gambusia holbrooki, from the onset of the heart rates (HRs) to just prior to parturition, was investigated using light cardiogram. The genetic sex of the embryos was post-verified using a sex-specific genetic marker. Results reveal that heart rates and resting time significantly increase (p < 0.05) with progressive embryonic development. Furthermore, both ventricular and atrial frequencies of female embryos were significantly higher (p < 0.05) than those of their male sibs at the corresponding developmental stages and remained so at all later developmental stages (p < 0.05). In concurrence, the heart rate and ventricular size of the adult females were also significantly (p < 0.05) higher and larger respectively than those of males. Collectively, the results suggest that the cardiac sex-dimorphism manifests as early as late-organogenesis and persists through adulthood in this species. These findings suggest that the cardiac measurements can be employed to non-invasively sex the developing embryos, well in advance of when their phenotypic sex is discernible. In addition, G. holbrooki could serve as a better model to study comparative vertebrate cardiovascular development as well as to investigate anthropogenic and climatic impacts on heart physiology of this species, that may be sex influenced.

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

“…Sex can affect physiology (Grilo et al, 2018;Millington and Rideout, 2018;Selmoni et al, 2019), development (Maitre et al, 2017;Gurley et al, 2018) and behaviour (Magurran and Garcia, 2000;Klemetsen et al, 2003;Pearse et al, 2019) well before gonad formation (Tsai et al, 2009;Mousavi et al, 2021). Divergent selection pressures may thus act on the sexes even at early developmental stages (Bolund et al, 2013;Schroeder et al, 2013;Forsman, 2018) leading to the development of distinct life history strategies and sometimes even sex-specific mortality (Badyaev et al, 2002;Wiklund et al, 2003;Husby et al, 2006; but see Dietrich et al, 2003;Altwegg et al, 2007).…”

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

“…In contrast, we were able to use HR (bpm) more broadly to stage the embryos and to distinguish sexspecific HR differences. 9 Nevertheless, it must be noted that the heart rate can be altered by temperature and measurements may require optimization for any given temperature. Also, increases in OVC and OVL in relation to body length were consistent with a multidimensional growth of embryonic organs (eg, brain and eyecup).…”

“…The HR (bpm) was measured from early segmentation (Se19) to parturition (Pa39) using light-cardiograms (LCG) as recently described for G. holbrooki. 9 Ten embryos per stage (ESe, MSe, LSe, EPh, LPh, and JPPa) were used for HR measurement at a constant temperature 25 ± 0.5 C.…”

“…Among vertebrates, fish exhibit the most diverse reproductive and developmental strategies. Of which, Poeciliids are one of the most fascinating, with regard to reproductive strategies, 1 embryo development, 2-6 evolution, 7 placentation, 8 sexual dimorphism, 9 sexual selection, 10 and genetic sex determination. 10,11 Apart from the short gestation of Poeciliids in general, the Gambusia holbrooki is an attractive model to study developmental pathways, evolution among taxa, and genetic sex determination and differentiation mechanisms.…”