Benefits of genetic rescue of a critically endangered subspecies from another subspecies outweigh risks: results of captive breeding trials

Small, isolated populations risk extinction through inbreeding depression, chance loss of beneficial variation, and reduced adaptability to changing environments. Genetic rescue via gene flow from genetically diverse sources is the most effective way to improve fitness of such populations. However, when the only potential source of immigrants is a different subspecies that diverged long ago and occupies a different environment, genetic rescue may lead to reduced fitness of admixed offspring through outbreeding depression or maladaptation. Test cases are needed to evaluate how to manage such potentially risky rescues to deliver enhanced population fitness. The helmeted honeyeater Lichenostomus melanops cassidix is a critically endangered subspecies of the yellow-tufted honeyeater. The sole remaining natural wild population experiences strong inbreeding depression for lifetime reproductive fitness. Captive genetic rescue trials are underway using a neighbouring subspecies, gippslandicus, which diverged from cassidix thousands of years ago and differs in morphology, mobility and preferred habitat. We evaluated short-term reproductive fitness for captive cassidix-cassidix pairs, first- and second-generation intersubspecific crosses and backcrosses to cassidix, while accounting for breeding season, sex, age at breeding, and wild/captive origin of each bird. Most admixed pair-types more readily engaged in breeding, raised more nestlings per nest, and had less male-biased chick sex-ratios than did cassidix-cassidix pairs, with negligible evidence of outbreeding depression. Continuing monitoring of fitness after releases into the wild is recommended, to ensure local adaptation is retained. With potentially riskier rescue increasingly becoming the only option for many populations, our study provides an encouraging test case.