It is widely acknowledged that genetic drift is an important source of variation in response to artificial directional selection. How large should a selection line be in order to reduce the effect of genetic drift to an acceptably low level?

This paper investigates two criteria that can be used to answer this question in relation to short-term response to selection. The first criterion is coefficient of variation of response, and the second criterion is chance of success, where a successful selection programme is one in which the observed response is greater than a certain proportion, β, of expected response.

For a simple mass selection programme with intensity i and heritability h2, the size of population required in order for the coefficient of variation of response to be γ after t generations, is approximately 2/(γih)2t, and the size required for the chance of success to be α after t generations is approximately 2{zα/(β−l)ih}2/t, where zα is the standard normal deviate corresponding to the probability α.

As an example, suppose it is required that after t generations the coefficient of variation of response be 10% or that there be a 90% chance of achieving at least 9/10 of expected response. Since ih ≤ 2 in most selection programmes, the size of population required is at least 50/t or 82/t respectively. If ih ≤ 1, the corresponding sizes are 200/t and 328/t.

Results are extended to enable the calculation of size of population required for any type of artificial directional selection programme, including those in which generations overlap.

The production of Inl+ recombinants was readily demonstrated in transductional crosses from Inl+ donor strains to Inl− recipient strains from different biotypes and biogroups. None of numerous crosses between different pairs of strains from biotypes 25 and 26 and the biogroup FIRN (biotypes 29, 30, 31 and 32) gave Inl+ recombinants suggesting that the inl mutation was present at the same intragenic site in all of these strains. Strains of the FIRN biogroup (Fim−Inl−Rha−Bxyl−) are thought to have descended by successive rha and fim mutations from an ancestral bacterium of biotype 25 (Fim+Inl−Rha+Bxyl−). The sites of the inl mutations in other Inl− biotypes (9, 10, 27 and 25hi) were independent and each was different from that in strains from biotypes 25, 26 and FIRN.

Cross-fertilizing stocks of Euplotes crassus, differing as to the ability to undergo autogamy, were subjected to breeding analysis. The resulting data are consistent with the interpretation that a single locus (a) with a pair of alleles (a+; a−) controls the ability to undergo autogamy. The two alleles express a simple dominance relationship in which the dominant allele (a+) permits expression of the autogamy trait. Cross-breeding experiments also suggest that the a locus is not linked to the mt locus which determines the mating-type trait.

In the mitochondrial and nuclear genetic background of the 777-3A strain (op1, ade1, α) a set of strains with different nutritional requirements and mating types, carrying the op1 mutation versus its wild-type OP1 allele was constructed. This makes possible the use of strains with the same genetic background in recombinational analysis of mitochondrial loci.