Variation in pollen and ovule fertility in varieties of cassava,and the effect of interspecific crossing on fertility

Records of the set of seeds obtained from intraspecific cassava crosses, from interspecific crosses between other Manihot species and cassava, and from backcrosses to cassava of three generations of interspecific hybrids are used to assess the variation in pollen and ovule fertility. It is concluded that the capacity of cassava varieties to set seeds has been reduced since they have evolved from non-tuberous wild forms and have been propagated vegetatively. Fertility was found to be very variable, and the capacity of the pollen of a variety to promote seed-set was not related to the fertility of the variety's female flowers. This situation would be expected if reduction in the fertilities of the two sexes were caused either by loss of balance in the genes controlling the functioning of the male gametophyte, or by unbalance of those concerned with the functioning of the female gametophyte or the tissues which nurture the embryo. Additional causes of sterility, probably including meiotic irregularities, appeared to operate in the F₁ interspecific hybrids, but such factors were probably not important after the first backcross generation. Manihot melanobasis, a wild form which is normally propagated by seed and whose relationship to cassava should be regarded as subspecific, contributed factors which enhanced the fertility of its hybrids with cassava. This form could be used in cassava breeding as a donor of seed fertility, but use could also be made of some existing varieties which still possess a moderately high capacity to set seed. It is desirable to select these as one parent when making difficult crosses.This paper is based almost entirely on the records of the late R. F. W. Nichols, who preceded the writer as cassava breeder with the East African Agriculture & Forestry Research Organisation, at Amani, Tanganyika.