CHROMOSOME PAIRING, FERTILITY, AND CROSSING BEHAVIOR OF HAPLOIDS OF TETRAPLOID ALFALFA, MEDICAGO SATIV A L.

Abstract

Chromosomes paired mainly as bivalents at metaphase in 15 haploids (2n=2x=16) from 10 unrelated stocks of cultivated alfalfa (2n=4x=32). The apparent high degree of chromosome homology and hence autopolyploid chromosome behavior exhibited by the comparatively large sample agreed with that previously reported in independent studies of three other 2x haploids. No gross structural rearrangements of the chromosomes were detected. Haploids produced as much as 50% stainable pollen, but it was only 1-2% germinable in vitro, and they were all effectively male sterile, even when derived from normal male fertile stocks. Two haploids had normal female fertility, whereas forty-eight others were reduced in female fertility in varying degrees. Only four were completely sterile. Since meiosis was normal, the reduction in fertility was due to factors operating postmeiotically. Haploids used as seed parents produced mostly diploid progeny in crosses with diploid M. sativa and M. falcata, and mostly tetraploid progeny in crosses with tetraploids. A few trisomics, other aneuploids, and a pentaploid were also produced. Haploids and diploids derived from them produced a higher frequency of unreduced female gametes than wild diploids. 2x haploids of alfalfa appear to be useful in new approaches to genetic and breeding research, since large numbers can he obtained, they are crossable with wild and derived diploids, and their germplasm can be transferred back to the tetraploid level by unreduced gametes.