Evolutionary consequences of seed banks and seed dispersal in Arabidopsis

Abstract

In most plant species, seeds after dispersing from the mother plant (primary dispersal) may undergo secondary dispersal, either in time by remaining dormant in the soil and forming a seed bank, or in space by movement to other locations. Seed bank and dispersal in space are bet-hedging strategies that minimize the extinction risk and aid to population stability and persistence in temporally variable environments. The ability to establish seed bank via depositing a fraction of seeds into the soil may also increase the potential of early stage adaptation into new habitats. A persistent seed bank preserves genetic diversity and buffers populations from the loss of genetic variants due to random drift. Although theoretical studies have highlighted the role of seed banks in elevating effective population size, little information exists regarding the evolutionary potential of seed banks in natural populations.

In this thesis I studied inter- and intraspecific variation in genetic composition of seed banks and its significance in natural populations of two closely related species in the genus Arabidopsis. A. thaliana is an annual self-compatible plant whereas A. lyrata is perennial and self-incompatible. The potential contribution of seed banks to effective population size (Ne), generation time, genetic variation, and population dynamics has been addressed. The pattern of dispersal over time and space was investigated by monitoring of natural populations over five consecutive years in A. thaliana. In addition, regional differences in seed bank, genetic composition and structure in A. lyrata were investigated between three contrasting regions in northern Europe.

The results showed that both species form persistent seed banks throughout their Norwegian distribution range. Seedling density was lower in A. lyrata than A. thaliana, as would be expected from a perennial. Moreover, the seed bank contributes to total effective population size in perennial A. lyrata, though not to the same extent as in A. thaliana. In A. lyrata both seed bank and above-ground individuals seemingly have a similar contribution to the total Ne.

Monitoring A. thaliana populations over multiple years revealed that in most populations two or more distinct multilocus genotypes were present, which often varied in frequency between years, leading to variation in within-population diversity. Although most of the distinct multilocus genotypes within each population were genetically similar, probably due to historical mutation and recombination events, in some cases multiple colonization events due to migration from other populations were evident. Many geographically closely situated populations shared common multilocus genotypes and expressed low differentiation compared to distant ones. The results show evidence dispersal in time, i.e., persistence of dormant or ungerminated seeds in ten populations in which 29% of seeds descended two or three years before present, on average. Additionally there are signs of seed and pollen immigration from other populations in almost one fifth of the studied cohorts, reflecting an effective migration rate of 1.8% per generation. Migration through pollen and seeds in A. thaliana is therefore common, and the seed bank plays, at the same time a substantial role in overall population dynamics.

Comparing natural populations of A. lyrata in different regions revealed that soil seed banks are either absent or small in Icelandic populations and average density of seed bank is 2.5 fold smaller than what was observed among Norwegian populations, though the overall differences between regions was not statistically significant. The level of genetic variation in Icelandic was similar to Swedish and significantly higher than what found in Norwegian populations. In addition population differentiation on Icelandic was significantly lower than what found in Swedish and Norwegian populations. When comparing similar distribution areas in the regions, the effect of habitat structure was found to be less important to explain the differences in genetic structuring. Immigration rate over time was similar between regions that show variable population differentiations. The results suggest that relatively low differentiation among Icelandic populations is more likely due to large historical effective population sizes compared to Scandinavian populations, rather than immigration per se.