‘Thawing’ of ‘frozen’ variation in an adventive, facultatively apomictic, clonal weed

Abstract

AbstractRadiation of adventive, sexually reproducing organisms into a new environment has been well documented, but less is understood about the patterns and processes of geographical radiation in adventive species with uniparental reproduction (parthenogenesis or apomixis), usually associated with ‘frozen’ variation within maternal lineages. In this study we have used Pilosella officinarum to test the hypothesis that sexual reproduction becomes important in the radiation of adventive facultatively apomictic species. Within its native Europe P. officinarum is an extremely variable species, with eight subspecies and a range of ploidy levels. Pentaploids are always apomictic. However, early chromosome analyses of New Zealand adventive populations, carried out during the 1980s, discovered only pentaploid, apomictic clones. Similar observations were made in an early 1990s study of the same sites. Since then, a range of ploidy levels, including aneuploids, and a plethora of morphological variation, has been recorded. In this paper, we use a combination of image analysis and chromosome counts to show that sexual reproduction has become an important avenue for adaptive radiation in New Zealand populations of P. officinarum. Current populations probably comprise complex among‐subspecies hybrids, and possibly among‐species hybrids, which are at least partially capable of sexual reproduction and various forms of back‐crossing. Somatic mutation may also play an important role in creating morphological variation. DNA fingerprinting did not contribute to this study because this technique failed to distinguish between morphological types observed in the field.