Late-acting self-incompatible system, preferential allogamy and delayed selfing in the heterostylous invasive populations of Ludwigia grandiflora subsp. hexapetala

Mating system influences local population genetic structure, effective size, offspring fitness and functional variation. Determining the respective importance of self- and cross-fertilization in hermaphroditic flowering plants is thus important to understand their ecology and evolution. The worldwide invasive species, Ludwigia grandiflora subsp. hexapetala (Lgh) presents two floral morphs: one self-compatible short-styled morph (S-morph) and one self-incompatible long-styled morph (L-morph). Most invasive populations worldwide are only composed of self-incompatible L-morphs, which questions the importance of sexual reproduction during the invasion. In this study, we identified the mating systems of western European experimental and natural populations of Lgh by comparing structural characteristics of pollen and style, by studying self- and cross-pollen tube elongations and the viability of the resulting seeds and seedlings in both morphs. Our results showed no differences in pollen shape and stigma surfaces among and between the two floral morphs. In the self-incompatible L-morph flowers, self-pollen tubes were stopped tardily, in the ovarian area, and were unable to fertilize the ovules. This first formal identification of a late-acting, prezygotic self-incompatible system in Ludwigia genus questions on the distribution of this mating system in the Myrtales order. In the self-compatible S-morph flowers, rarer in worldwide invasive populations, self-pollen always succeeded to self-fertilize the ovules that nearly all developed into viable seedlings. However, cross-pollen tubes always elongated faster than self-pollen tubes. S-morph individuals may thus advantage preferential allogamy over selfing when cross-pollen is available despite its self-compatibility. As expected in late-acting self-incompatible systems, L-morph flowers authorised 0.2‰ of selfed seeds during the uppermost flowering season, that increased to 1‰ at the end of the flowering season. Such delayed selfing resulting in a significant quantity of viable floating seeds may contribute to the local regeneration, seed bank and propagation of the L-morph, which may explain its invasion success worldwide. Management plans of Lgh would gain to consider the mixed mating system we identified.