Breeding systems and relationships of the Cerastium tomentosum group

A programme of artificial hybridization, involving seven species of the Cerastium tomentosum group at four levels of ploidy from tetraploid to 16-ploid, produced viable F1 seed in 22 (49 %) of the 45 combinations. Eleven (24 %) of these gave rise to mature F1 plants, of which three were fertile. Greatest success was with the octoploid × tetraploid and octoploid × octoploid crosses, of which 50 % gave mature plants, including all three fertile F1 combinations. Of the latter C. tomentosum × C. grandiflorum is octoploid, but C. tomentosum × C. biebersteinii and × C. gibraltaricum are hexaploids (2n = 54), a chromosome number not known in any wild plants of this group but evidently one capable of producing viable gametes despite apparently irregular meioses. Hybrids between the C. tomentosum group and species of the C. alpinum, C. arvense, C. banaticum and C. latifolium groups were equally successful in producing viable F1 seed and mature F1 plants. The octoploid C. tomentosum × C. arvense hybrid was fertile, again despite irregular meiosis, but later generations can probably develop a perfectly regular meiosis as occurs in wild hybrids between these species that occur in south-eastern England (where C. tomentosum has become naturalised). In nature the species studied rarely hybridise due to geographical isolation. Most plants are self-compatible but strongly protandrous, and many exhibit varying degrees of male sterility. It is suggested that the five above ‘groups’ should be recognised as a single undivided taxon (subsection Cerastium).