Animal Population Structure Under Close Inbreeding: The Land Snail Rumina in Southern France

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Abstract
The pulmonate land snail R. decollata of the Mediterranean region is a complex of monogenic strains maintained, at least in part, by a breeding system of faculative self-fertilization. Colonies may consist of 1 monogenic strain or a variable combination of the 2. Interstrain hybridization was detected in a study of the composition of 24 colonies in a 2.5 acre area in Montpellier [France]; the proportion of individuals having mixed genomes varied from 0% to 36.8% with a mean of 8.6%. The mean value of Wright''s fixation index for the colonies was 0.7748, and the frequency of outcrossing was crudely estimated at 0.150. The rate of outcrossing was frequency dependent, being highest in colonies in which the dark-bodied strain was decidedly in the minority. Hybrid individuals produce viable and fertile young, and they are normally fecund under laboratory conditions. The fact that the strains remain monogenic despite the potential for introgression suggests strong coadaptation of their respective genomes. Strongly inbred species apparently retain much genetic variability, distributed largely among monomorphic lines and occasionally released through outcrossing.

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