Evidence for the existence of endosperm balance number in the true clovers (Trifolium spp.)

The endosperm balance number (EBN) hypothesis was first advanced to explain results from interspecific crosses in Solanum and later in Impatiens. According to the EBN hypothesis, normal endosperm development following intra- or inter-specific crosses depends on having a ratio of two EBNs from the female to one EBN from the male in the endosperm tissue. EBNs may differ among related species. Successful hybrids can be obtained between species with the same EBN. The ploidy level of an individual species can be varied to modify its EBN, making it cross compatible with a species sharing its modified EBN. Interspecific crosses within Trifolium have been limited and difficult. Crosses reported in the literature, including evidence from our own study, suggest that EBN is operating in Trifolium and have been used to assign EBN numbers to some clover species. The use of 2n eggs enabled two species, differing in EBN, to be crossed. An understanding of the EBN mechanism that operates in Trifolium should make successf...