Sexual selection maintains whole-body chiral dimorphism in snails.
Although the vast majority of higher animals are fixed for one chiral morph or another, the cause for this directionality is known in only a few cases. In snails, for example, rare individuals of the opposite coil are unable to mate with individuals of normal coil, so directionality is maintained by frequency-dependent selection. The snail subgenus Amphidromus presents an unexplained exception, because dextral (D) and sinistral (S) individuals occur sympatrically in roughly equal proportions (so-called 'antisymmetry') in most species. Here we show that in Amphidromus there is sexual selection for dimorphism, rather than selection for monomorphism. We found that matings between D and S individuals occur more frequently than expected by chance. Anatomical investigations showed that the chirality of the spermatophore and the female reproductive tract probably allow a greater fecundity in such inter-chiral matings. Computer simulation confirms that under these circumstances, sustained dimorphism is the expected outcome.
Schilthuizen
M
M
Institute for Tropical Biology and Conservation, Universiti Malaysia Sabah, Kota Kinabalu, Malaysia. schilthuizen@naturalis.nl
Craze
P G
PG
Cabanban
A S
AS
Davison
A
A
Stone
J
J
Gittenberger
E
E
Scott
B J
BJ
D000818:Animals; D019521:Body Patterning; D003198:Computer Simulation; D005260:Female; D008297:Male; D052586:Mating Preference, Animal; D012727:Sex Characteristics; D012908:Snails / Q000033:anatomy & histology / Q000502:physiology*; D013093:Spermatogonia / Q000166:cytology
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