Hybridisation and genetic diversity in introduced Mimulus (Phrymaceae)

Abstract

Hybridisation among taxa with different ploidy levels is often associated with hybrid sterility. Clonal reproduction can stabilise these hybrids, but pervasive clonality may have a profound impact on the distribution of genetic diversity in natural populations. Here we investigate a widespread triploid taxon resulting from hybridisation between diploid Mimulus guttatus and tetraploid Mimulus luteus, two species that were introduced into the United Kingdom (UK) in the nineteenth century. This hybrid, Mimulus x robertsii, is largely sterile but capable of prolific vegetative propagation and has been recorded in the wild since 1872. We surveyed 40 Mimulus populations from localities across the UK to examine the current incidence of hybrids, and selected seventeen populations for genetic analysis using codominant markers. Cluster analyses revealed two main groups of genetically distinct individuals, corresponding to either diploid (M. guttatus) or polyploid (M. luteus and M. x robertsii) samples. Triploid hybrids were found in around 50% of sampled sites, sometimes coexisting with one of the parental species (M. guttatus). The other parent, M. luteus, was restricted to a single locality. Individual populations of M. x robertsii were genetically variable, containing multiple, highly heterozygous clones, with the majority of genetic variation distributed among- rather than within populations. Our findings demonstrate that this largely sterile, clonal taxon can preserve non-negligible amounts of genetic variation. The presence of genetically variable hybrid populations may provide the material for the continued success of asexual taxa in diverse environments.

Figure 1

Locations of populations sampled across the UK. Closed circles indicate populations for which genetic analysis was carried out.

Figure 2

Results of the DAPCs showing the genetic clustering of 300 Mimulus spp. individuals from 17 populations across Great Britain. (a) Individual probability of membership to either of two genetic clusters (k*=2) identified in the DAPC analysis. Cluster 1 (blue) is associated with diploid individuals (M. guttatus), whereas cluster 2 (red) represents polyploid individuals (M. x robertsii and M. luteus s.l.). The x axis shows the population of origin from north to south, with population codes as given in Table 4. (b) Discriminant analysis using the first 15 principal components shows a clear ability to distinguish the two genetic clusters. The x axis shows the values of the first discriminant function for each individual (vertical bars), and the y axis indicates the smoothed density of observations. (c, d) Analysis of cluster 2 samples only. (c) Individual probabilities of membership to either of four genetic clusters (k*=4) identified in the DAPC analysis. Notice that the tetraploid individuals (M. luteus s.l.) from population COL are not uniquely separated from triploid (M. x robertsii) individuals in other populations (BAL and KES). (d) Discriminant analysis of cluster 2 samples showing the values of the first discriminant function. Colours correspond to the four genetic clusters in (c).

Figure 3

Relationships among individuals sampled as inferred by principal coordinate analysis of binary data using Dice's genetic distance (see Materials and methods). The goodness of fit of the first two principal components was 43%. Symbols: circles=diploid (M. guttatus), squares=triploid (M. x robertsii), triangles=tetraploid (M. x smithii=M. luteus var. luteus × M. luteus var. variegatus). Geographic populations are represented with different colours and identified with their respective population codes. For mixed populations, codes are shown for both diploid and polyploid samples.

Figure 4

Neighbour-joining tree summarising the relationship among studied populations based on Dice genetic distance. The ploidy level is indicated for each population (2 × , 3 × , 4 × ).