Studies on the physiology and genetics of the rotifer, Asplanchna. III. Results of outcrossing, selfing, and selection

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Abstract
Sympatric populations of Asplanchna brightwelli and A. girodi do not hybridize, so that these two morphological species represent distinct biological species or syngens. Populations of A. brightwelli from Tennessee and Indiana may be crossed to give viable F1, F2, and backcross generations. These two widely separated populations may thus be assigned to a single biological species. A program of repeated selfing with selection for high clonal viability has been carried out with A. brightwelli. Clonal viability declined with the first selfing, then rose with succeeding generations of selfing and selection, to or above the viability of clones obtained by inbreeding within the parental mixed stock. Selfing also led to an increase in the homogeneity of the growth rates of groups of sibling clones obtained by selfing, suggesting increased homozygosity in the selfed, selected lines. The results of the selfing and selection experiments suggest that wild stocks of Asplanchna are heterozygous for a number of recessive detrimental and lethal genes affecting the growth rate and viability of clones. The heterozygosity of the natural populations, together with the failure of geographically isolated populations of a morphological species to become reproductively isolated, suggests that these rotifers are not extreme inbreeders. It is suggested that the maintenance of recessive detrimentals in these rotifers, in which the males are haploid, may indicate that the phenotype of a male is determined to a large extent by the genotype of its mother. Such maternal effects would protect detrimentals from selection in the male offspring of heterozygous females.