Kākāpō reproduction: Identification of steroid receptors and oestrogenic activity in native flora

Abstract

Kākāpō (Strigops habroptilus) are a critically endangered parrot species endemic to New Zealand that exhibit a reproductive strategy linked to “masting” years. Crucial to their survival is increased reproductive success. It has been hypothesised that their pattern of reproduction is synchronised with a steroidal “trigger” present in plants during intensive masting. If this hypothesis is valid, then Kākāpō and other closely related NZ parrots might be receptive to these masting plants in a manner different from that of other avian species.

The aims of this study were firstly, to identify whether unique amino acid motifs were present in the ligand binding domains (LBD) for the steroid receptors oestrogen receptor α (ER-α), oestrogen receptor β (ER-β), androgen receptor (AR) and progesterone receptor (PR) for the New Zealand parrots Kākāpō, Kākā, Kākāriki and Kea. These LBD amino acid sequences were compared with those in an Australian parrot, the Cockatiel, as well as in Chicken and Japanese Quail. Moreover, the role of these amino acid changes on the binding of ligands (both the native ligand and other steroidogenic compounds) for ER-α was assessed by in silico modelling by comparing the most favourable binding position of the ligands in the three-dimensional structure of Kākāpō ER-α with that of human ER-α. The second aim was to test extracts of New Zealand native plants known to be a food source for Kākāpō for their seasonal variation in oestrogenic activity and hence possible involvement in the reproductive cycle of Kākāpō.

The LBD for ER-β and AR of the parrot species (New Zealand and Australian) displayed 100% identity whilst those for ER-α and PR had variations. When the parrot sequences were compared with those for other avian species and human, there were a number of amino acid differences present, with the greatest disparity present in the LBD of ER-α. From in silico modelling studies, the amino acid substitutions in this receptor were predicted to have an indirect influence on the binding position of both 17β-oestradiol and a number of phytoestrogens through changes in the structure of this region. Consequently, this could have an effect on the binding affinity of certain plant chemicals for ER-α and thus their potency.

Plants extracts from the foliage and fruit of native New Zealand species were screened for oestrogenic activity in a yeast bioassay transfected with the human isoform of ER-α. The results from these experiments indicated that all the plants tested (excluding Dacrycarpus dacrydioides) exhibited detectable oestrogenic activity and a number also displayed variable and sometimes seasonal trends in activity.

In summary, the results in this study identified ER-α LBD amino acid motifs unique to New Zealand parrots. For the other steroid receptors, the ER-β LBD had a three amino acid combination of M, V and L that was only present in the parrot species whilst both the AR and PR LBD displayed much greater inter-species conservation. These changes, particularly those in the ER-α LBD, may influence the binding of steroidogenic compounds, known to be present in the plants consumed by Kākāpō as demonstrated by in silico modelling. Thus, this research provides evidence that the diet of Kākāpō may influence their reproductive pattern. However, it remains to be determined whether the steroidogenic potency of these plant extracts is modified when interacting with the unique LBD of native NZ parrots.