The accumulation of genetic diversity within a canopy‐stored seed bank

Abstract

Many plants regenerate after fire from a canopy-stored seed bank, in which seed are housed in fire resistant confructescences (cones) that remain on maternal plants. This strategy would be favoured if plants accumulate a sufficiently large and genetically diverse seed bank during interfire intervals. We use a 16-year demographic study and surveys of microsatellite variation to quantify and explain the rate of accumulation of genetic diversity within the canopy seed bank of the shrub Banksia spinulosa. Flowering and fruit set were highly variable. An initial sample in 1991 of 354 reproductively mature plants generated 426 cones over 16 years, of which only 55 cones from 40 maternal plants persisted until 2005. By genotyping seed from these 55 cones we demonstrated that genetic diversity accumulated rapidly within the seed bank. Resampling revealed that diversity was determined by the number, not the age, of cones. Cones were widely distributed among plants, outcrossing rates were high (mean t(m) = 1.00 +/- 0.04) and biparental inbreeding low. Adults displayed little evidence of isolation by distance and the genotypic diversity of seed cohorts was independent of the density of neighbouring potential sires. We therefore estimate that within at least 13 individual years the number of cones produced per year (14-63) would have contained 100% of the adult genetic diversity. We conclude that a highly outcrossed mating system and relatively widespread pollen dispersal ensure the rapid development of a genetically diverse and spatially and temporally homogeneous seed bank.

Is the post-disturbance composition of a plant population determined by selection for outcrossed seedlings or by the composition of the seedbank?

“…These findings support the results of a growing number of studies that have compared genetic diversity across plant life stages (see Honnay et al, 2008 for review), and more specifically, the results of a very limited number of studies that have compared either the soil-or canopystored seedbanks of species occurring in fire-prone habitats (Barrett et al, 2005;Premoli and Kitzberger, 2005;Ayre et al, 2009Ayre et al, , 2010. Our findings likely reflect the presence of long-lived, soil-stored seedbanks that, together with extended inter-fire periods, would allow the build-up of large persistent seedbanks that capture the progeny of a number of years of mating (England et al, 2001), and potentially the contributions of multiple generations of adult plants .…”

“…Similar fine-scale spatial structure typically at distances of 5-10 m has been reported for other Proteaceae in fire-prone habitats (Ayre et al, , 2010Forrest et al, 2011;Ritchie and Krauss, 2012;Ayre and O'Brien, 2013) and has been argued to reflect the formation of clusters of closely related seedlings in seed shadows of pre-fire adults. Intriguingly, spatial clustering of seedlings is often reported over large spatial scales than is the case for adults (for example, Ayre et al, 2009).…”

Increased fire frequency promotes stronger spatial genetic structure and natural selection at regional and local scales in Pinus halepensis Mill

“…Similar findings have been reported for the fire-adapted obligate seeder Ulex parviflorus (Moreira et al, 2014). Genetic diversity can be preserved through frequent fires because of adaptations to fire, such as an early age of first flowering and a diverse canopy seed bank due to serotiny (Templeton and Levin, 1979;Ayre et al, 2010), and because of high levels of gene flow Shohami and Nathan, 2014). In the Proteaceae species Banksia hookeriana, for example, genetic diversity in a 9-year-old canopy seed bank accumulated rapidly: the first reproduction event after fire already accounted for >80 % of the genetic diversity detected (based on the Shannon index I) and genetic variation between seed crop years was small (Barrett et al, 2005).…”

Limiting inbreeding in disjunct and isolated populations of a woody shrub

“…Although there were significant correlated matings, the number of pollen parents per seed parent was moderate and the majority of successful pollen came from more distant (>10 m), relatively unrelated sources within populations. High levels of outcrossing have been reported previously in many members of the Proteaceae (Holmes et al 2009;Krauss et al 2009;Ayre et al 2010;Llorens et al 2011). In H. oldfieldii, the correlation of selfing among loci estimates suggested that all the selfing detected was due to biparental mating rather than selfpollination, suggesting there may be a mechanism to prevent self-pollination in this species.…”