Seed morphological traits as a tool to quantify variation maintained in ex situ collections: a case study in Pinus torreyana

Understanding the within- and among-population distribution of trait variation within seed collections may provide a means to approximate standing genetic variation and inform plant conservation. This study aimed to estimate population- and family-level seed trait variability for existing seed collections of Torrey pine (Pinus torreyana), and to use these data to guide sampling of future collections. We quantified variation in 14 seed morphological traits and seedling emergence within and among Torrey pine populations. Using a simulation-based approach, we used estimates of within-population variance to assess the number of maternal families required to capture 95 % of trait variation within each existing seed collection. Substantial structure was observed both within and among Torrey pine populations, with island and mainland seeds varying in seed size and seed coat thickness. Despite morphological differences, seedling emergence was similar across populations. Simulations revealed that 83 % and 71 % of all maternal families within island and mainland seed collections respectively needed to be resampled to capture 95 % of seed trait variation within existing collections. From a conservation perspective, our results indicate that to optimize genetic diversity captured in Torrey pine seed collections, maximizing the number of maternal families sampled within each population will be necessary.

Foliar Water Uptake in the Needles of Pinus Torreyana

In semi-arid climate systems that encounter low clouds or fog, many plants have adapted to capture atmospheric moisture on the foliar surface. The natural habitat of Pinus torreyana, the Torrey pine, is restricted to two locales in coastal Southern California that experience substantial fog and low clouds during the dry months of a Mediterranean climate. In this study, droplet contact angles were used to assess surface wettability, and droplet uptake was used to measure the foliar water uptake rate along a needle. We demonstrated that there are only remnants of epicuticular wax on the needle. The results showed that the needle surface of P. torreyana, including under the base sheath, is hydrophilic and capable of direct water uptake. There are small spatial gradients in the uptake pattern, but the variability is high and the statistical significance is not strong. However, the decrease in uptake at higher contact angles can be described by an ordinary linear regression (r2 = 0.45, p = 4 x 10-8). On average, the foliar uptake of the adaxial surface of a one-year-old needle is 2.88 ± 0.60 x 10-2 mg cm–2s–1. In comparison, the uptake rates by the adaxial surfaces of four local broadleaf plants are between 0.05 and 0.6 of that of P. torreyana.

Reduced representation sequencing to understand the evolutionary history of Torrey pine (Pinus torreyana Parry) with implications for rare species conservation

Understanding the contribution of neutral and adaptive evolutionary processes to population differences is often necessary for better informed management and conservation of rare species. In this study, we focused on Pinus torreyana Parry (Torrey pine), one of the world's rarest pines, endemic to one island and one mainland population in California. Small population size, low genetic diversity, and susceptibility to abiotic and biotic stresses suggest Torrey pine may benefit from inter-population genetic rescue to preserve the species' evolutionary potential. We leveraged reduced representation sequencing to tease apart the respective contributions of stochastic and deterministic evolutionary processes to population differentiation. We applied these data to model spatial and temporal demographic changes in effective population sizes and genetic connectivity, to assess loci possibly under selection, and evaluate genetic rescue as a potential conservation strategy. Overall, we observed exceedingly low standing variation reflecting consistently low effective population sizes across time and limited genetic differentiation suggesting maintenance of gene flow following divergence. However, genome scans identified more than 2000 SNPs candidates for divergent selection. Combined with previous observations indicating population phenotypic differentiation, this indicates that natural selection has likely contributed to population genetic differences. Thus, while reduced genetic diversity, small effective population size, and genetic connectivity between populations suggest genetic rescue could mitigate the adverse effect of rarity, divergent selection between populations indicates that genetic mixing could disrupt adaptation. Further work evaluating the fitness consequences of inter-population admixture is necessary to empirically evaluate the trade-offs associated with genetic rescue in Torrey pine.

RNA-Seq Reveals Adaptive Genetic Potential of the Rare Torrey Pine (Pinus torreyana) in the Face of Ips Bark Beetle Outbreaks

The ability of tree species to adapt to water stress and increased frequency of bark beetle outbreaks with climate change may increase with population size and standing genetic variation, calling into question the resilience of small, rare plant populations. The Torrey pine (Pinus torreyana) is a rare, genetically depauperate conifer that occurs naturally in a mainland and island population in southern California. Due to recent declines in the mainland population coinciding with drought and Ips paraconfusus bark beetle outbreaks, the species would benefit from an assessment of adaptive genetic diversity. Here, we use RNA-Seq to survey gene-coding diversity across 40 individuals to 1) characterize patterns of genetic diversity in the species and 2) test for genetic differentiation between trees that succumbed to beetle attack or survived following an outbreak. Consistent with previous studies, we found few genetic variants, with most SNPs occurring as fixed differences between populations. However, we found structure within the mainland and polymorphisms segregating in both populations. Interestingly, we found differentiation in genotypes between attacked and surviving trees, 11 SNPs associated with survival status, and an enrichment of defense-related functions among the top 10% of SNPs. While low diversity suggests limited adaptive capacity, genetic associations with survival in functionally relevant genes suggest adaptive potential for bark beetle defense. This initial study prompts future research to explore the genetic basis of putative resistance and suggests conservation efforts should protect surviving genotypes and the full spectrum of genetic diversity across populations to preserve the evolutionary potential of the species.

Seed morphology and emergence variability in a conservation collection of Pinus torreyana

ABSTRACTPREMISEOptimizing the amount of genetic diversity captured in seed collections is a long-standing objective of ex situ conservation. Particularly for rare species where limited genetic information is available, it poses a significant challenge. However, understanding the within and among population distribution of trait variation within seed collections may provide a means to approximate standing genetic variation and inform conservation efforts.METHODSWe quantified seed morphological variation and seedling emergence both within and among populations for existing seed collections of Torrey pine (Pinus torreyana), a critically endangered pine endemic to California. Using a simulation-based approach, we used estimates of within-population variance to estimate the number of maternal families required to captured 95% of trait variation within each existing seed collection.KEY RESULTSOn average, 21% and 22% of seed morphological variation in Torrey pine was explained by population origin and maternal family within populations respectively. Despite significant morphological differences, seedling emergence was similar across populations. Simulations revealed that 80% and 68% of all maternal families within island and mainland seed collection respectively needed to be resampled to ensure 95% of seed trait variation within existing collections was captured.CONCLUSIONSOverall, these results suggest that substantial structure exists for seed morphological traits both within and between populations. From a conservation perspective, this indicates that to optimize genetic diversity captured in Torrey pine ex situ conservation collections, maximizing the number of maternal families sampled within each population will be necessary.