Population dynamics at a diploid-polyploid contact zone in Mercurialis annua L.

Abstract

Hybrid zones offer ‘natural laboratories’ for studying the origin, maintenance and demise of species, serving as examples of evolution in action. Understanding the processes that mediate the simultaneous invasions and extinctions of a hybrid zone is therefore essential to understanding the range limits of species and the maintenance of their genetic identity. Populations of diploid (2n = 16) and hexaploid (2n = 48) Mercuralis annua L. s.l. (Euphorbiaceae) have migrated from glacial refugia towards a zone of secondary contact that now exists in north-east Spain. Rapid movement of this zone in favour of the diploids over the past 50 years has been attributed to an interaction of endogenous and exogenous selection, and has lead to the formation of a narrow ‘tension hybrid zone’ that limits the spatial mixing of these lineages. This work systematically disentangles the factors underlying the stability of the diploidhexaploid contact zone in its current position in the north-east Spain to understand the fate of this hybrid zone if it follows its current trajectory into what I have characterised as a low-density trough. I examine the importance of demographic factors such as founding size, reproductive assurance in polyploids, and differences in the rate of seed bank formation for the persistence of both lineages in this area. This provides an estimate of ambient rates of extinction in this zone for each lineage, independent of their interactions with one another, and highlights their susceptibility to density-dependent processes at each life history stage. I then examine asymmetries in hybridisation dynamics between lineages in experimental field populations of controlled density as well as in naturally occurring mixed populations in this zone, assessing the effect of biased gene flow of diploids on the outcome of these apparently rare contact events. I also refine an earlier form of the mass-action pollen pool model to account for the spatial context of individuals and their individual-based contribution to the pollen pool. Using this enables a prediction of hybridisation rates in the field for given mixed assemblages. Results are interpreted in the context of tension zone dynamics and the long-term fate of sexual system variation in this species complex. The identification of long-run negative growth rates in both lineages, and the fact that a small fraction of sites remained by the end of a four-year time course of study, are both indicative of a region that represents a low-density trough into which the diploid-hexaploid contact zone will settle if movement continues along its density-dependent trajectory.This thesis is not currently available in ORA