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

Abstract

Seedbanks are expected to buffer populations against disturbances, such as fire, that could alter the genetic composition of smaller, ephemeral adult populations. However, seedling genotypes may be influenced by the spatially heterogeneous nature of both the seedbank and the disturbance (for example, germination may vary with local disturbance) and also by selection acting on germination and post-germination performance. We used microsatellite-DNA surveys of seedlings emerging from the soilstored seedbanks of Grevillea macleayana after wildfire to compare diversity and spatial structure in seedlings and adults, and through resampling of the seedling data set, to determine whether the resultant adult population reflected the effects of selection or random seedling mortality. The large post-fire seedling cohorts captured the full allelic diversity of the pre-fire adult population. However, we found a mismatch in the genotypic structure of adults and seedlings. Seedlings displayed larger heterozygous deficits than adults; however, over the ensuing 11 years, seedling heterozygosity eventually matched values for the pre-fire adults. Increasing heterozygosity among adults has generally been attributed to heterosis and/or reduction in Wahlund effects via self-thinning. Resampling of early post-fire seedlings to generate samples of equivalent size to survivors at 11 years showed that increases in heterozygosity must be driven by selection favouring outcrossed seed. This finding is important in an evolutionary context but also has implications for the restoration of natural or managed populations where a seedbank is a viable source of recruits.

Anthropogenic fragmentation may not alter pre‐existing patterns of genetic diversity and differentiation in perennial shrubs

“…Self‐compatible species are much less likely to experience reduced mate availability due to low variation at SI loci or to show inbreeding depression in small populations (Leimu et al., ), as the genetic load may have been purged following a long history of inbreeding. However, self‐compatible plants may still show considerable effects of inbreeding depression at different life history stages (González‐Varo & Traveset, ; Michalski & Durka, ; Roberts et al., ), which could represent a mechanism to maintain genetic diversity. Although a decline in genetic diversity may reduce evolutionary potential and hence the resilience of populations to climate change, small populations of G. caleyi , like those of some other highly fragmented species such as the Mediterranean Myrtus communis (González‐Varo, Nora, & Aparicio, ), are more likely to face extinction from demographic factors and edge‐related disturbance (Llorens, ) before genetic factors can play a role.…”

Genetic and epigenetic divergence between disturbed and undisturbed subpopulations of a Mediterranean shrub: a 20‐year field experiment

“…2008; Roberts et al. 2014). Rapid phenotypic changes following disturbance have been related to shifts in genetic composition of populations (Scheiner and Teeri 1987).…”

“…2008; Roberts et al. 2014), but few studies have documented postdisturbance genetic shifts (Dolan et al. 2008; Banks et al.…”

“…2008; Roberts et al. 2014). The potential of disturbance to cause rapid genetic change in plant populations and the possible mechanisms involved, however, remain essentially unexplored to date (Banks et al.…”

Paternity analysis reveals wide pollen dispersal and high multiple paternity in a small isolated population of the bird-pollinated Eucalyptus caesia (Myrtaceae)

“…For example, self-pollination can result in depressed seed set and plant growth compared with controlled outcrossed pollinations in some eucalypts (Hardner and Potts, 1995). However, multiple mating and pre-or post-dispersal selection against inbred individuals has fitness benefits that may buffer the loss of genetic diversity in small populations (Harder and Potts, 1997;Pannell and Labouche, 2013;Roberts et al, 2014). Rare, localised endemics with limited dispersability may have evolved pollination strategies in response to prolonged isolation to maintain or enhance polyandry and genetic diversity (the James Effect hypothesis; sensu Hopper, 2009).…”