Mimicry genes reduce pre-adult survival rate in Papilio polytes: A possible new mechanism for maintaining female-limited polymorphism in Batesian mimicry

Batesian mimicry, in which harmless organisms resemble unpalatable or
harmful species, is a well-studied adaptation for predation avoidance. The
females of some Batesian mimic species comprise mimetic and non-mimetic
individuals. Mimetic females of such polymorphic species clearly have a
selective advantage due to decreased predation pressure, but the selective
forces that maintain non-mimetic females in a population remain unclear.
In the swallowtail butterfly, Papilio polytes, female polymorphism is
controlled by the H (non-mimetic) and h (mimetic)
alleles at a single autosomal locus. Here, we examined if the dominant
H allele has a deleterious effect on the pre-adult survival rate
(egg-to-adult emergence rate). We repeated an assortative mating-like
treatment—i.e. breeding of males and females whose mothers had the same
phenotype (mimetic or non-mimetic)—for three consecutive generations,
while avoiding inbreeding. Results showed that pre-adult survival rate
decreased over generations only in lines derived from mothers with the
mimetic phenotype (hereafter, mimetic-assorted lines). This lowered
survival was due to an increased mortality at the final instar larval
stage and the pupal stages. Interestingly, the pre-adult mortality in the
mimetic-assorted lines seemed to be associated with a male-biased sex
ratio at adult emergence. These results suggest that the dominant
H allele displays a mildly deleterious effect that is expressed
more strongly in females and homozygous individuals than in heterozygous
individuals. We propose that this cost of mimicry in larval and pupal
stages contributes to the maintenance of female-limited polymorphism in P.
polytes.