Abstract
In plants, apomixis results in the production of clonal offspring via seed and can provide reproductive assurance for isolated individuals. However, many apomicts require pollination to develop functional endosperm for successful seed set (pseudogamy) and therefore risk pollination-limitation, particularly in self-incompatible species that require heterospecific pollen. We used microsatellite paternity analysis and hand pollinations to investigate pollen-limitation in Sorbus subcuneata, a threatened endemic tree that co-occurs with its congener, S. admonitor. We confirmed that S. subcuneata is an obligate pseudogamous apomict, but open-pollinated flowers rarely produced seed (flower-to-seed conversion < 1%) even though they rapidly accumulated pollen on their stigmas. Manual heterospecific pollination by S. admonitor resulted in a high flower-to-seed conversion rate (65%), however, we estimate that the ratio of self: heterospecific pollination in open-pollinated flowers was at least 22:1. Despite the efficacy of heterospecific pollination, the contribution of S. admonitor trees to paternity in seed from open-pollinated flowers of S. subcuneata decreased rapidly with the spatial separation between paternal and maternal trees. Conservation efforts aimed at maintaining species with this breeding system must therefore manage the congeners in tandem which will also maintain the potential for rare heterospecific fertilisation that typically cause rapid diversification in these lineages.
Highlights
In plants, apomixis results in the production of clonal offspring via seed and can provide reproductive assurance for isolated individuals
We confirmed that triploid S. subcuneata is an obligate pseudogamous apomict, because hand-pollinations demonstrated that seed production required pollination (Table 1) and microsatellite analysis revealed that all of the 95 embryos studied had microsatellite phenotypes that were identical to both their maternal tree and the S. subcuneata reference samples shown in Table S1, Supplementary Information
We found that S. subcuneata set seed readily through hand-pollination with congeneric S. admonitor pollen, with a flower-to-seed conversion rate of 65% (Table 1)
Summary
Apomixis results in the production of clonal offspring via seed and can provide reproductive assurance for isolated individuals. Many apomicts require pollination to develop functional endosperm for successful seed set (pseudogamy) and risk pollination-limitation, in self-incompatible species that require heterospecific pollen. Many apomicts are pseudogamous, which means that they require pollen to develop functional endosperm for the maturation of their otherwise clonal seed. For species with a pseudogamous apomictic (PA) breeding system, the requirement for pollination may limit seed set just as it often does in sexual species[7]. In the Rosaceae, many polyploid pseudogamous apomicts are self-compatible 8, which enables seed set through autogamous (within flower) or geitonogamous (within individual) pollen transfer. Some PA species in the Rosaceae are self-incompatible (SI)[9], which means that they require heterospecific pollination by congeners. We investigated the performance of this perplexing breeding system in a woodland community of a rare member of the Rosaceae, a putatively self-incompatible triploid Sorbus species
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