INHERITANCE OF THE GYNODIOECY OF THYME (THYMUS VULGARIS L.)

Gynodioecy, defined by the coexistence of male fertile (mf) hermaphroditic and male sterile (ms) individuals in natural populations of a species, is typical for thyme. Male sterility is of uppermost interest for plant breeders to facilitate controlled pollination of parental components of hybrid cultivars. The aim of the investigation was to deepen the knowledge on the genetics of the fertility type determination and to find genetic sources of cytoplasmic ms and mf types which are qualified to maintain male sterility in the progeny (maintainers). In 2003, 61 F₁-populations were generated by test crossings of 5 ms and 19 mf plants and in addition 19 I₁-populations were produced by selfing the mf parents. In 2004, the offspring, 4593 plants in all, were screened during full flowering and their fertility types were categorised into eight classes, considering the varying morphology of the stamen. The gradual differences between the fertility classes indicate that several different genetic factors contribute to the expression of the fertility types. Reciprocal crosses of male fertile plants revealed the existence of cytoplasms with different restorer properties. The differing share of ms plants in the individual progenies of one and the same ms mother plant fertilised by different mf pollinators is evidence for different restorer factors in the nuclear genome of mf plants. The ratio of complete ms in 61 hybrid populations varied between 0 and 100%. The plants of ten F₁-populations were all ms. The sterility could be maintained by four mf genotypes. No general model of nucleo-cytoplasmic inheritance of the fertility types could be inferred due to the complex interaction of possibly several cytoplasmic types and different nuclear restorer genes. Despite the complex interaction of a variety of nuclear and cytoplasmic loci, crossing progeny with 100% ms were found in specific combinations of test crosses providing the needed ms lines with their maintainers for hybrid breeding.