An analysis of mating structure in populations of the annual sea rocket, Cakile maritima (Brassicaceae)

Abstract

Cakile maritima is an introduced brassicaceous annual plant, now occurring widely along the Australian coastline. While variable, populations of C. maritima often show high levels of infection by a fungal pathogen, Alternaria brassicicola. As part of a larger metapopulation study of host–pathogen dynamics in this system, an isozyme survey of seven populations of Cakile was carried out along the south coast of New South Wales to investigate mating structure. Given the possibility of self-incompatibility, a glasshouse crossing experiment was also carried out to investigate the potential for selfing. The results from the crossing study showed that C. maritima is basically an obligate outcrosser, but that self-compatible individuals also occur at low frequency, providing some flexibility in the mating system. Similarly, analysis of the isozyme data showed a preponderance of cross fertilisation, but with significant low levels of selfing in two populations. There were no correlations between outcrossing rates and population size or density, although there was evidence of restricted mating at the individual level (low paternal diversity within seed from a given mother). However, this did not translate into biparental inbreeding, even in very small populations, most likely due to incompatibility leading to negative assortative mating.