Australian researchers find that the benefits of genetic mixing between subspecies designed to improve genetic health and fitness outweigh risks to the Critically Endangered Helmeted Honeyeater
A team of conservation biologists based at Monash University are collaborating with Zoos Victoria to save the Critically Endangered Helmeted Honeyeater, Lichenostomus melanops cassidix (referred to as cassidix here), from extinction. This small black-and-yellow songbird is distinguished by its characteristic ‘helmet’ of golden feathers atop its head, and is one of four living subspecies of the Yellow-Tufted Honeyeater, Lichenostomus melanops. The Helmeted Honeyeater is endemic to Victoria and was formally recognized as its official bird in 1971.
The Helmeted Honeyeater lives in dense swamp forests that lie alongside streams. Although this riparian habitat was originally widespread, it was drained and cleared for agriculture by white European colonists starting in the 1880s. Despite these challenges, the Helmeted Honeyeater still managed to cling to life in a few remnants of its habitat until a large wildfire swept through the area in 1983, destroying all but one tiny population of the birds at the Yellingbo Nature Conservation Reserve.
Predictably, the habitat at Yellingbo has become increasingly degraded. A variety of water-loving native trees, such as Mountain Swamp Gum Eucalyptus, Eucalyptus camphora, are dying due to altered hydrology in the area, and invasive weeds, such as blackberries, are rapidly moving in, thereby preventing restoration of the swamp habitat naturally. Introduced species, such as deer, and overabundant native species, particularly wallabies, are adding to the woes faced by the few remaining Helmeted Honeyeaters by competing with them for scarce resources.
“This habitat degradation resulted in strong decline of helmeted honeyeaters to ~50 birds by the 1990s,” lead author of the study, evolutionary biologist Alexandra Pavlova, a Senior Research Fellow at Monash University, told me in email. Dr Pavlova also serves as the co-lead of the Wildlife Genetic Management Group at Monash University and as a member of the Helmeted Honeyeater Recovery Team.
For these reasons, Helmeted Honeyeaters are listed as Critically Endangered by the International Union for Conservation of Nature (IUCN), which is the global authority on the status of the natural world and the measures needed to safeguard it. Thanks to intensive conservation and captive breeding efforts, the Yellingbo population of Helmeted Honeyeaters is currently estimated to number roughly 250 individuals. But things are not going well for these songbirds.
“At such small population sizes, loss of genetic diversity and loss of fitness (survival and reproduction) due to inbreeding become inevitable, which means that the population is likely to dwindle to extinction even if the habitat is restored,” Dr Pavlova pointed out. “We estimate that by 2018 the most inbred birds produced only 1/10th of the total number of offspring in a lifetime compared to the least inbred birds.”
Clearly, the highly inbred cassidix subspecies was in poor genetic health. It appeared the only reasonable recourse was to develop a genetic rescue project to save it from extinction.
To initiate this genetic rescue project for the Yellingbo Helmeted Honeyeaters, Dr Pavlova gathered together a team of collaborators from Monash University and Zoos Victoria. Together, they realized that they may potentially improve the Yellingbo birds’ genetic health by breeding them with their closest living relatives, the neighboring Gippsland Yellow-tufted Honeyeater, Lichenostomus melanops gippslandicus (see Graphical Abstract), a subspecies that diverged from cassidix thousands of years ago and differs from them in morphology, mobility and preferred habitat.
The adaptive differences between subspecies makes gene flow between them to be viewed as risky and thus, best avoided. However, gene flow from some outside source population is essential to save Helmeted Honeyeaters.
“Genetic rescue via gene flow from genetically diverse sources is the most effective way to improve fitness of isolated inbred populations,” Dr Pavlova explained. “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 or maladaptation.”
To learn whether an admixture of genes between these two subspecies would be helpful or harmful to cassidix, a breeding trial was conducted over five years’ time by Zoos Victoria’s Healesville Sanctuary.
“The Helmeted Honeyeater is most visibly distinguished from its closest relative — the Gippsland Yellow-Tufted Honeyeater — by the size of its ‘helmet’ of long, erect crown feathers,” Dr Pavlova reported.
“Anecdotally, the helmet of first-generation (F1) Helmeted Honeyeater X gippsland crossed offspring may be slightly smaller than in the Helmeted Honeyeater parent, but longer than in the gippsland parent. In the backcross (offspring of the F1 and helmeted honeyeater) it is almost indistinguishable from the Helmeted Honeyeater parent.”
How will Dr Pavlova and collaborators know whether this genetic admixture between the two subspecies is progressing as planned?
“During the genetic rescue, we will monitor visible and invisible (genetic) characteristics and make any adjustments required to the management actions,” Dr Pavlova stated. “Genetic rescue necessarily needs to change some aspects of the genetic composition of the rescued population, for the better.”
Already, short-term reproductive fitness has been assessed for captive pairings of ‘pure’ Helmeted Honeyeater pairs, first- and second-generation inter-subspecific crosses, and backcrosses to the Helmeted Honeyeater, taking into consideration sex, breeding season, age at breeding, and wild or captive origin of the parents.
What have Dr Pavlova and collaborators observed with regards to ‘pure’ versus admixed individuals?
“We have observed that, compared to ‘pure’ pairs, most admixed pair-types more readily formed a pair and built a nest, raised more nestlings, and had chicks that were less male-biased, with no signs of reduced fitness,” Dr Pavlova replied in email. Dr Pavlova also recommended that fitness monitoring should continue after release of the birds into the wild to ensure the population maintains its adaptations to the local environment.
What is the most important lesson to be learnt from this study?
“Our study is significant because it provides a positive example of how potentially ‘risky’ rescue options could help improve population genetic health and fitness. This is particularly important, because risky gene flow sources are becoming the only choice for many populations,” Dr Pavlova said.
It appears clear that genetic rescue is a powerful conservation tool if used carefully to conserve a wide variety of Endangered species. Are there plans to use genetic rescue to help other Critically Endangered species?
“Genetic management of plants and animals has been used extensively for decades. Genetic rescue — deliberate crossing of different populations as a conservation strategy to improve their genetic health — is more recent, but there are around 50 clear cases of which we are aware,” Dr Pavlova replied to me in email. “These include four bird species other than Helmeted Honeyeater: Bearded Vulture, Mauritius Pink Pigeon, Greater Prairie Chicken, and Red-cockaded Woodpecker.”
Dr Pavlova noted that increasing the genetic diversity of declining populations is equivalent to equipping them with tools to overcome inbreeding and to evolve new adaptations in response to changing environments.
“Genetic management is not only effective in its own right, but it also enhances the positive impacts of other conservation actions,” Dr Pavlova continued. “For example, alleviating pressure on populations via habitat restoration is usually crucial, but will be less effective if the genetic health of the population is not sufficient to make use of the improved environment.”
But what about species, such as the migratory orange-bellied parrot, Neophema chrysogaster (that I’ve written about here, here and here), without any subspecies to rescue them, that have declined to the point where there are only a handful of individuals remaining?
“In some desperate cases when the species is only represented by a few related individuals, other closely related species will have to be considered,” Dr Pavlova replied to me in email. “This may be riskier, because genomes of these species may have become incompatible over the course of long divergence. However, the benefit to population health may still outweigh the risk of lost fitness due to outbreeding. One example for consideration is the rescue of the endangered orange-bellied parrot using the blue-winged parrot: with no closer relative available for rescue, we should try risky rescue while it is not too late.”
Although orange-bellied and blue-winged parrots represent two distinct species, they are each other’s closest living relatives and, as such, may have maintained a genetic connection through occasional hydridization.
“Natural hybridization among species is not uncommon, and such cases are a valuable source of information about what might be worthwhile considering in conservation actions.”
Alexandra Pavlova, Sara Petrovic, Katherine A. Harrisson, Karina Cartwright, Elizabeth Dobson, Laura L. Hurley, Meagan Lane, Michael J.L. Magrath, Kimberly A. Miller, Bruce Quin, Monique Winterhoff, Jian D.L. Yen, and Paul Sunnucks (2023). Benefits of genetic rescue of a critically endangered subspecies from another subspecies outweigh risks: Results of captive breeding trials, Biological Conservation 284:110203 | doi:10.1016/j.biocon.2023.110203
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