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Factors Affecting Efficiency of Hybridization and Selection in Cassava¹

An analysis of genetic variability in several populations of cassava (Manihot esculenta Crantz) revealed that it is a highly heterozygous species. This species is monoecious, and no genetic or physiological barrier to prevent self-fertilization was found. Both self and crosspollination occurred naturally. Thirty meters of separation between populations was sufficient to ensure genetic isolation. The average yield of selfed lines was about half the average yield of the parental genotypes. The degree of inbreeding depression varied greatly in different genotypes. Average yields of populations derived from open pollination of male-fertile genotypes were lower than the average yields of populations derived from the controlled pollinations of the same genotypes. In male.sterile genotypes, no yield difference was observed between the open-pollinated progenies and those from the controlled pollinations. Controlled pollinations are recommended as a general method of hybridization in cassava. If a breeder chooses open pollinations, use of male steriles is recommended. Correlations of root yield and harvest index (proportion of root weight to total plant weight) between the seedling plant and the same genotype when planted with stem cuttings were high. Root yield in a single-row trial was not correlated with root yield in a replicated population trial, but harvest index in the single.row trial was significantly correlated with the root yield in the population trial. Intergenotypic competition caused a yield difference of more than 100%, but there was no significance in harvest-index difference. Harvest index was highly heritable, and it is an excellent selection character.

Key Words: Pollinations • Male-sterility • Inbreeding depression • Heterozygosity • Competition • Heritability • Harvest index

² Plant breeder, research assistant, germplasm superintendent and postgraduate intern, respectively.

Received for publication August 26, 1976.

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