Distribution and concentration of maternal progesterone in the yolk of Greater Rhea eggs (Rhea americana)

Abstract

Progesterone is the most concentrated maternal yolk steroid characterized to date in birds; however, no information about it is available in ratite eggs. We collected freshly laid eggs from zoo-housed Greater Rhea females (Rhea americana) bred under similar rearing conditions during two breeding seasons to characterize concentration and distribution of maternal yolk progesterone. After high-performance liquid chromatography analysis, yolk hormone was measured using a commercial electrochemiluminescence immunoassay. Progesterone concentrations were found to vary significantly among the yolk layers, supporting a follicular origin for this steroid in Greater Rhea eggs. Additionally, highly similar mean absolute yolk progesterone concentrations were detected between 2013 and 2015 breeding seasons (1,332.98 ± 82.59 and 1,313.59 ± 85.19 ng/g, respectively). These values are also comparable to those found in some domestic carinate species. Findings suggest that at population level, when rearing conditions are similar, mean absolute yolk maternal progesterone concentrations also appear bounded. Future research on the factors and mechanisms that regulate progesterone deposition in Greater Rhea eggs is needed to better understand whether its levels depend on different rearing conditions.

Yolk corticosterone and progesterone levels in Greater Rhea (Rhea americana) eggs vary in a changing social environment

“…In the Greater Rhea , we previously found that yolk hormone deposition varies among eggs of different captive populations and could influence the chicks' physiology and behavior (Della Costa et al, 2019). We also found that yolk hormone levels did not vary when the rearing conditions remain similar thorough the years (Della Costa et al, 2017). However, it is unknown whether Greater Rhea females could modify yolk hormone deposition when the social environment changes.…”

“…Other factors related to the females' state (e.g., age, physical condition, and past experiences) have changed over the years. However, we previously reported that yolk corticosterone and progesterone did not vary between the laying seasons when the Greater Rhea females are maintained under similar captive conditions (Della Costa, 2018; Della Costa et al, 2017). Thus, it is not expected that yolk hormone deposition varies with females aging within reproductive years.…”

“…Yolk progesterone concentration was determined using a commercial Electrochemiluminescence Immunoassay Kit (Elecsys Progesterone II from ROCHE), previously validated for the yolk of Greater Rhea (Della Costa et al, 2017). Although the cross‐reaction of the antibody with other metabolites is very low (<2%, Della Costa et al, 2017), we will refer to immunoreactive substances detected by the antibody as “immunoreactive progesterone”. Hormone quantification was performed in a COBAS 6001 equipment with a module for immunoassay e 601 (HITACHI High Technology Corporation‐ROCHE Diagnostic GmbH).…”