Rare interspecific hybridization at the diploid level in Ranunculus sect. Batrachium

Hybridization and polyploidization are significant evolutionary mechanisms. Allopolyploidy, the formation of polyploids through hybridization between distinct species followed by whole-genome duplication, is common mainly in plants, where it facilitates rapid speciation and ecological diversification. Hybridization between diploid species often results in F1 hybrids with reduced fertility due to irregular meiosis. However, subsequent genome duplication can restore fertility by providing homologous chromosome pairs. Understanding the initial hybridization between diploids is therefore crucial for elucidating the early stages of allopolyploid formation. Ranunculus sect. Batrachium is known for the frequent occurrence of hybrids and polyploids, many of which are allopolyploid. Surprisingly, no unequivocal natural hybrid between diploid species has been discovered until recently in spite of the large body of evidence on interspecific hybridization in this group. This study provides evidence for hybridization at the diploid level, based on an integrated morphological, cytological and molecular approach. A new interspecific hybrid R. circinatus × R. rionii was detected in two flooded sand pits in the Czech Republic and described as R. ×limnophilus. Maternal inheritance patterns indicated independent, reciprocal origin of the hybrids at each locality. The ecological and historical context of the hybrid’s emergence is discussed, including colonization of a newly formed habitat that allowed coexistence of the two parental species, their hybridization and the subsequent vegetative persistence of the hybrid despite its sterility. Similar conditions may have prevailed during the early postglacial period, facilitating the formation of allopolyploids, which today constitute the majority of species diversity in this group. A review and categorization of the biotypes that constitute the complex diversity in Ranunculus sect. Batrachium is also provided.