Genetic rescue strategies for small isolated populations of endangered marsupial species

"To save a species, sometimes you have to shake up its gene pool" - this practical wisdom drives genetic rescue, the deliberate introduction of individuals from genetically distinct populations into inbred isolates to restore heterozygosity and fitness. This research evaluated the genetic status and rescue potential of four populations of the endangered brush-tailed rock-wallaby (Petrogale penicillata) using microsatellite analysis at 16 loci and mitochondrial D-loop sequencing. Samples (tissue biopsies and scats) were collected from a large mainland population (n = 47), a medium island population (n = 38), and two small isolated populations (n = 24 and n = 18) through collaboration with Australian conservation agencies, with laboratory analysis at the Bengal Academy of Life Sciences, Kolkata, from February to November 2023. Expected heterozygosity ranged from 0.62 in the large mainland population to 0.24 in the smallest isolate. Inbreeding coefficients (FIS) reached 0.31 in small isolate B, and allelic richness was 53.2% lower than the mainland baseline. Simulated genetic rescue scenarios using the software Vortex showed that introducing two mainland individuals per generation for five generations would raise heterozygosity by 38.7% and reduce extinction probability from 72% to 18% over 50 years. These results provide a genetic evidence base for implementing rescue translocations in critically inbred marsupial populations.

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Impact of Genetic Rescue on Extinction RiskClose ×

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He: Large mainland vs Medium island vs Small isolate A vs Small isolate BClose ×

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AR: Large mainland vs Medium island vs Small isolate A vs Small isolate BClose ×

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Introduction

"A species is not truly lost when the last individual dies - it begins to disappear when the last genetically distinct population can no longer sustain itself." This idea, though seldom attributed to a single source, captures why conservation geneticists worry about small, isolated populations long before they reach the brink of demographic extinction [1]. Genetic diversity is the raw material for adaptation, and populations that lose it through drift and inbreeding become trapped in a fitness decline that accelerates with each generation [2]. Genetic rescue - the introduction of unrelated individuals into inbred populations - has been successfully demonstrated in Florida panthers, Swedish adders, and Australian mountain pygmy possums [3, 4]. But applying this strategy to marsupials, whose reproductive biology differs from placental mammals, requires careful evaluation of genetic compatibility, disease risk, and outbreeding depression potential [5].

The brush-tailed rock-wallaby (Petrogale penicillata) was once widespread across southeastern Australia but has declined to fragmented populations due to predation by foxes and habitat loss [6]. Several small colonies numbering fewer than 30 individuals persist on predator-free islands and fenced reserves, but their genetic status hasn't been comprehensively assessed. This research genotyped four populations spanning a range of sizes and isolation levels to quantify inbreeding, model population viability, and evaluate genetic rescue scenarios.

Material And Methods

Material Genetic samples were obtained from four P. penicillata populations: (1) large mainland (n = 47, Wollemi National Park, NSW), (2) medium island (n = 38, Kangaroo Island, SA), (3) small isolate A (n = 24, Grampians National Park, VIC), and (4) small isolate B (n = 18, NAAS Rating: 4.82 IJBS 2025; 7(5): 182-184 Upasana Bhandarkar Department of Conservation Biology, Bengal Academy of Life Sciences, Kolkata, West Bengal, India Lavanya Balasubramanian Department of Conservation Biology, Bengal Academy of Life Sciences, Kolkata, West Bengal, India Upasana Bhandarkar Department of Conservation Biology, Bengal Academy of Life Sciences, Kolkata, West Bengal, India https://www.biologyjournal.net conducted in Vortex 10.5.5 using demographic parameters from published life-table data for P. penicillata. Three rescue scenarios were modeled over 50 years: (1) no intervention, (2) introduction of 2 mainland individuals per generation (every 4 years), and (3) introduction of 4 individuals per generation. Extinction probability was defined as population falling below 5 individuals. Each scenario ran 1,000 iterations.

Results

Table 1: Genetic diversity metrics across four P. penicillata populations (mean ± SD across 16 microsatellite loci) Shoalhaven Gorge, NSW). Tissue biopsies (ear punches from trapped animals) and fecal samples were collected by field teams from partner agencies during 2022-2023 trapping programs. DNA extraction, microsatellite genotyping, and analysis were performed at the Bengal Academy of Life Sciences, Kolkata, between February and November 2023. DNA was extracted using the DNeasy Blood and Tissue Kit (Qiagen).

Population	He	Ho	AR	FIS
Large mainland	0.62±0.11	0.58±0.09	6.8±1.4	0.06±0.03
Medium island	0.48±0.09	0.42±0.08	4.7±1.1	0.13±0.05
Small isolate A	0.28±0.07	0.22±0.06	3.2±0.8	0.21±0.06
Small isolate B	0.24±0.06	0.18±0.05	2.9±0.7	0.31±0.07

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Genetic diversity metrics across four P. penicillata

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Sixteen microsatellite loci previously developed for Petrogale species were amplified and scored on an ABI 3730xl capillary sequencer. Ethics approval: Bengal Academy of Life Sciences Animal Research Ethics Committee (Protocol BALS-AREC-2022- 015, November 2022).

Methods

Allele frequencies, expected heterozygosity (He), observed heterozygosity (Ho), allelic richness (AR, rarefied to minimum sample size), and FIS were calculated in GenAlEx 6.5 and FSTAT 2.9. Hardy-Weinberg equilibrium was tested per locus per population. Population structure was assessed with STRUCTURE 2.3 (K = 1-6, 100,000 burnin, 500,000 MCMC). Mitochondrial D-loop sequences (487 bp) were aligned in MEGA 11 and used to construct a median-joining haplotype network.

Population viability analysis (PVA) was Population He Ho AR FIS Large mainland 0.62±0.11 0.58±0.09 6.8±1.4 0.06±0.03 Medium island 0.48±0.09 0.42±0.08 4.7±1.1 0.13±0.05 Small isolate A 0.28±0.07 0.22±0.06 3.2±0.8 0.21±0.06 Small isolate B 0.24±0.06 0.18±0.05 2.9±0.7 0.31±0.07 Genetic diversity declined sharply with population size and isolation (Table 1). The two small isolates retained less than half the heterozygosity and allelic richness of the mainland population. FIS values confirmed substantial inbreeding, reaching 0.31 in the smallest population. Fig 1: Box plot of expected heterozygosity across four P. penicillata populations and a simulated post-rescue scenario, showing the potential for genetic restoration through managed translocation Fig 2: Distribution of individual inbreeding coefficients in small isolate B before (outer ring) and after simulated genetic rescue (inner ring), showing shift toward lower inbreeding categories https://www.biologyjournal.net 7.

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Distribution of individual inbreeding coefficients in small isolate B before (outer ring) and after simulated genetic rescue (inner ring),

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Box plot of expected heterozygosity across four P. penicillata populations and a simulated post-rescue scenario, showing the potential

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Conservation Genetics. 2012;13(6):1625-1634. Vortex simulations showed that small isolate B faced a 72% extinction probability over 50 years without intervention. Introducing 2 mainland animals per generation reduced this to 18%, while 4 animals per generation brought it to 6%. Heterozygosity in the rescued population was projected to reach 0.41 after five rounds of introduction, a 70.8% increase from the pre-rescue baseline.

Discussion

The genetic erosion documented in these small wallaby populations mirrors patterns seen in other isolated marsupials, including the mountain pygmy possum and the woylie [7, 8]. What makes these data actionable is the Vortex modeling, which translates abstract genetic metrics into concrete management scenarios. The finding that just two immigrants per generation can cut extinction risk from 72% to 18% provides conservation managers with a feasible and relatively low-cost intervention [9]. The risk of outbreeding depression - where mixing divergent genomes produces offspring less fit than either parent - is a legitimate concern but appears low for P. penicillata.

STRUCTURE analysis assigned all four populations to a single genetic cluster (K = 1 was the best-supported model), and mitochondrial divergence was only 1.3% between the most distant populations. This indicates recent common ancestry and minimal risk of genomic incompatibility [10, 11].

Conclusion

Small isolated populations of P. penicillata have lost over half their genetic diversity relative to mainland counterparts, with inbreeding coefficients reaching levels associated with fitness decline. Simulation modeling showed that introducing 2-4 mainland individuals per generation could restore heterozygosity and dramatically reduce extinction risk over a 50-year horizon. The low genetic divergence between populations minimizes outbreeding depression risk. These results provide quantitative support for implementing genetic rescue translocations as a conservation tool for this and similarly endangered marsupial species [12, 13, 14].

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