Medicago murex with 2n=16 and 2n=14 chromosome complements

Abstract

Contrary to the generally held view, M. murex Willd. was found to be of 2n=14 constitution in the great majority of accessions. The 2n= 16 type, though distributed sparsely over a wide area, is most likely a relic. Chromosome studies at pachytene indicate that the 2n=14 type probably arose from the 2n=16 type by translocation of the chromosomal material of the smallest chromosome in the 2n=16 complement to one of the longer chromosomes, with loss of the centromere of the small chromosome. The total chromosome lengths of the two types thus remained not significantly different, which is in line with the few small morphological differences which were detectable between the two types. Of considerably phylogenetic interest was the finding of complete inter-sterility of the two M. murex types. Neither M. aculeata 2n= 16 nor M. constricta 2n=14 could be crossed with M. murex of the same chromosome number. Inviable hybrids were obtained on crossing M. murex 2n= 16 with M. turbinata 2n=16. The spiny vs. spineless pod character was investigated in the 2n=16 type and found to be inherited on a monofactorial basis.