Relationship in annual spceies of Medicago. III. The complex M. littoralis Rhode-M. truncatula Gaertn

Abstract

A programme of interspecific hybridization, involving species from subsections Pachyspirae, Leptospirae, and Rotatae of section Spirocarpos, resulted in only one successful combination out of the 20 attempted.

Viable hybrids were obtained with ease in crosses between different accessions of M. littoralis and M. truncatula. The morphology, fertility, and cytological behaviour of interspecific hybrids involving seven strains of M. truncatula and three strains of M. littoralis was compared with intraspecific hybrids of each species. With the exception of those with N.2829, which was differentiated from the other strains by one chromosome translocation, intraspecific hybrids of M. truncatula were fully fertile.

Considerable pollen sterility and reduced seed set were observed in intraspecific crosses of M. littoralis, but there was no evidence of chromosomal rearrangements.

The morphology of the hybrids between M. littoralis and M. truncatula was intermediate between the parents. Chlorophyll deficiency in seedlings and adult plants, dwarfism, and other morphological irregularities were observed in F2 plants. In some FT₁s failure of pairing, multivalent formation at metaphase I, and other irregularities at meiosis indicated chromosome rearrangements. Up to three independent translocations and one inversion in the hybrid combinations and pronounced sterility were observed in both FT₁s and FT₂s. The pollen sterility of the hybrids could be explained in several instances by structural differences, but these do not account completely for the results obtained.

Inheritance studies of three possible strain markers indicated simple monohybrid segregation in most intraspecific crosses, but in the interspecific and partially fertile intraspecific crosses, ratios were disturbed and phenotypes abnormal.

The agronomic implications of these studies are discussed.