Cytological and Developmental Studies of Hybrids between Medicago Sativa and a Diploid Form of M. Falcata.

Received July 21, 1939 HE two species, Medicago satiua and M . falcata, are commonly T considered as varieties of one highly polymorphic species, since they intercross readily (WALDRON 1920, TYSDAL and WESTOVER 1937) and all intergradations between the two main types may be found. M . falcata, however, exists in both the diploid and tetraploid form (FRYER 1930)~ and it is only as the tetraploid that it crosses readily with the tetraploid M . sativa. Heretofore, no successful cross between the diploid M . falcata and M . satiua has been reported. The present paper describes two triploid hybrids formed when the diploid form of M . falcata was the male parent, and several tetraploid hybrids formed when it was the female parent in crosses with M . satiua. Histological studies were made in an endeavor to find the origin of the tetraploid hybrids and an explanation for the frequent failure of development of the triploid hybrids. Conditions in the developing ovules following crosses between plants of different chromosome number in Medicago are similar to those in Avena (KIHARA and NISHIYAMA 1932), Triticum (WAKAKUWA 1934, BOYES and THOMPSON 1937) and Nicotiana (KOSTOFF 1930). Fertilization occurs, but apparently the relation between the nuclear and cellular divisions in the endosperm is upset, so that many wrinkled, inviable seeds are formed when the female has the lower chromosome number but fewer, plumper, more viable seeds when the female has the higher chromosome number. The causes of the failure and the reciprocal cross differences in Medicago seem to lie in the endosperm as in the plants previously studied, though Medicago seeds normally have no endosperm a t maturity.