CYTOGENETIC BEHAVIOUR OF ALFALFA HYBRIDS FROM TETRAPLOID BY DIPLOID CROSSES

Abstract

Most cytological and genetic studies have supported the hypothesis that common tetraploid alfalfa, M e d i c n g o sntiva L. is primarily an autoploid. Cytological studies by Ledingham ( 1940), JulCn ( 1944) and Armstrong ( 1954) indicated that at various polyploid levels, the chromosomes interpair freely as if they were multiples of a single basic genome. Tetrasomic inheritance has been the rule in alfalfa since Stanford (1951) reported the tetrasomic inheritance of purple vs. white flower colour. Nevertheless, the cytological behaviour of the normal tetraploid at meiosis is not typical of autoploids in that multiple chromosome configurations occur with a low frequency. More recently, Cleveland and Stanford ( 1959) observed in newly produced M e d i c a g o autotetraploids (doubled diploid forms) that the quadrivalent frequency was also low, being only 34% of the possible maximum. This was still significantly higher than the 2 quadrivalents of normal tetraploid M. sntiva. They suggested that structural differences within the chromosome complement or a genic system may control chromosome association in M. sativa. Oldemeyer and Brink ( 195 3) compared the fertility of similar induced autotetraploids with the tetraploid variety Cossack. The induced tetraploids were less fertile than Cossack but when crossed to Cossack the F1 was just as fertile as the variety. They favoured the hypothesis of an autotetraploid origin of common alfalfa. The alternative hypothesis is that it is a segmental alloploid produced by the doubling of natural hybrids between closely related diploid species. T h e two most agronomically important species of alfalfa, M. sntiva L. and M. falcata L. are found most frequently as tetraploids although diploid forms exist. Interspecific crosses between the diploid and tetraploid forms occasionally set seed. When the diploid was used as the female parent, Ledingham (1940j, Armstrong (1954) and Cleveland and Stanford (1959) obtained only tetraploid hybrids while Nilan ( 1951) observed two triploids and four tetraploids. The reciprocal crosses have more frequently yielded both tetraploids and triploids. This paper describes the breeding behaviour and cytology of triploid and tetraploid hybrids from tetraploid by diploid crosses. The inheritance of male sterility in the tetraploid hybrids is reported and the results are considered in relation to theories on the control of multiple chromosome associations.