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Conserving the leafless Bossiaea
A team of geneticists have helped to clarify taxonomy and conservation planning for seven leafless Bossiaea species, writes Chantelle Doyle, Restoration Genomics Outreach Coordinator.
When geneticists from the Research Centre for Ecosystem Resilience (ReCER) started working with the NSW Saving Our Species (SoS) team to investigate the quirky group of peas known as the ‘leafless Bossiaea’, the project bounced in more than one unexpected direction.
Genetic profiling of seven species enabled identification of a possible new subspecies, collapse of two species into one and ongoing research to ask, at what point do we intentionally mix isolated plant populations to reduce inbreeding, and could this mixing include hybridisation? Leafless Bossiaea stand out for their flattened, photosynthetic stems and highly reduced leaves that have evolved into tiny scales, giving the appearance of being leafless.
There are 78 recognised species of Bossiaea, from shrubs to small trees. While most species have traditional stems, leaves, and the characteristic pea flowers that rely on pollinators to transfer pollen, a subset of species along Australia’s east coast has evolved differently. These plants have replaced leaves with flattened, photosynthetic stems called cladodes.
The evolution of aphyllous or leafless plants may be an adaptation to stressful environments. The absence or reduction of leaves reduces water loss and heat stress, through reducing the total surface area used for photosynthesis and respiration.
Having less surface area is also theorised to reduce the nutrients required to grow and maintain leaves which means leafless species can grow in nutrientpoor soils. Leaflessness occurs in many plant families across the globe, such as Cactacaeae (Cactus) and Euphorbiacaea (Euphorbias). These plant families are often associated with hot, arid, nutrient-poor environments.
The challenge of classification
Taxonomists classify genera and species primarily by floral structure, but Bossiaea is tricky as its floral traits are often too similar. Instead, leaf, hair (indumentum), stipule, and fruit traits are used. Leafless Bossiaea are particularly challenging, having scales instead of classic leaves. This group is also the most widely distributed, with the 12 known species ranging from far north Queensland to Victoria and Tasmania.
Key traits like stem scales, flower arrangement, bracts, growth form, and location help classify them. Morphological differences and distinct locations suggest isolated populations might be unique species – but is this supported by genetics? Working on a world leading program with the NSW SoS, ReCER geneticists have been able to review the classification of seven leafless Bossiaea, five of which are threatened with extinction.
Genetic tools make these conclusions feasible
Eilish McMaster, a geneticist with the Conservation Genomics team at ReCER, analysed the leafless Bossiaea species using genetic markers called Single Nucleotide Polymorphisms (SNPs). These tiny variations in DNA are often shared among related individuals and help identify distinct populations and species.
The analysis revealed that many leafless Bossiaea species have limited gene flow, with seeds and pollen rarely traveling less than one kilometre. This isolation suggests that populations rely heavily on clonal reproduction and self-pollination, raising concerns about their genetic health.
Encouragingly, the research also identified a previously unknown population of Bossiaea vombata and suggested that two other species should be considered a single species.
One of the most intriguing findings involves two species, Bossiaea fragrans and Bossiaea milesiae, which are separated by 300km. Despite their geographic isolation, genetic data suggests they are closely related and may represent two subspecies of a single species. The scientists suggest trialling crossbreeding between the redescribed subspecies to assess its viability for conservation, and to test the biological isolation between groups.
Both subspecies are struggling with unpredictable seed production, which could be due to inbreeding within isolated populations. Plants, like animals, are often healthiest when they mate with unrelated individuals, but geographic isolation can limit options.
If inbreeding is identified as a contributing factor to the limited seed production, a solution would be to test mixing the subspecies. Given the genetic evidence suggesting these lineages are closely related, they should be able to mate successfully. This would increase the genetic diversity of both populations, potentially enhancing their resilience and protecting them from long-term extinction.
Hybridisation provides a powerful potential pathway to conserve species with limited gene pools. However, within our current Australian environmental laws, threatened species conserved through adding new genetics from other closely related species may not be protected because they are hybrids.
Once a species reaches the point of ‘functional extinction’ – when it can no longer reproduce due to inbreeding –hybridisation may become the only means of preserving both biodiversity and the unique genes of the species.
Fortunately, neither Bossiaea subspecies is at this critical juncture, and more research is needed to identify the factors limiting seed production. Nonetheless, identifying opportunities for long-term conservation, even in the worst-case scenarios, is a vital aspect of proactive decision-making that we must begin to prioritise to ensure the survival of these species for future generations.
