Ovarian ultrasound analysis for developing temporal and spatially explicit hormone regimens for induced ovulation and egg deposition in the tiger salamander (Ambystoma tigrinum)

Abstract

Conservation breeding programs (CBPs) for threatened salamander species depend on reliable reproduction. We followed ovarian maturation and oviposition in eastern tiger salamander (Ambystoma tigrinum) females treated with exogenous hormone regimens developed within a framework of grading their initial follicular stages based on oocyte size and echogenicity (G0 = little/no development; G1 = minor development; G2 = moderate development; G3 = extensive development). The exogenous hormones used in this study included human chorionic gonadotropin (hCG) and gonadotropin releasing hormone (GnRH), where lower concentrations of hCG were used as a priming hormone compared to the ovulatory dose, which included both hCG and GnRH. The three hormone treatment regimens consisted of: (1) an extended prime plus ovulatory dose; (2) a short prime plus an ovulatory dose; and (3) an ovulatory dose only. The three exogenous hormone regimens were trialed for optimal spawning success based on ovarian ultrasound grades 1–3, in which higher doses of priming hormones were administered to salamanders with less developed follicles to stimulate oogenesis, prior to a final ovulatory dose. The three exogenous hormone regimens and three ovarian grades were used in a 3 × 3 factorial design, with n = 6 females for each factorial combination, and efficacy assessed by ovarian grade and/or exogenous hormone regimen on proportion of animals laying eggs, egg numbers, and latency to egg deposition. The salamanders in the most advanced initial stage of follicular development (G3) were most likely to ovulate and lay eggs and those with low follicular development (G1) the least likely. Initial follicular stages also affected the egg numbers, with G3 females laying the most eggs and G1 laying the fewest. Moreover, we found a significant interaction effect of exogenous hormone treatment and ovarian grade, in which G1 females required the greatest numbers and levels of exogenous hormone doses to induce oviposition and G3 the least. These findings show that ultrasound can be used with Ambystomid species to develop optimal spatial and temporal hormone regimens for ovulation and oviposition, depending on follicular maturation, thereby enhancing the efficiency of CBPs, improving animal welfare, and advancing genetic diversity management.