Biology Reference
In-Depth Information
the population. h is is not trivial because natural selection acts to re-
duce genetic variation. If a trait is very important for an organism, and
one heritable phenotypic variant is better with respect to survival and
reproduction, the less favorable variant will eventually be eliminated or
driven to very low frequencies in the population, taking with it the ge-
netic variation for the trait. Any genetic-variation-based model of the
evolution of polyandry must have a mechanism to maintain genetic
variation against directional selection. Second, it must show how in-
creasing numbers of mates af ects the genotypic composition of colo-
nies and colony i tness (survival and reproduction).
4.2 Sex Determination and Polyandry
h e sex-determination hypothesis for the evolution of polyandry is, in my
opinion, a front-runner for honey bees and also for some other social Hy-
menoptera. It proposes that polyandry in honey bees may be explained
by the ef ects of the sex-determination system on colony survival and
reproductive success. Honey bees are haplodiploid. Males have just one
set of chromosomes, while females have two. Because males and fe-
males have dif erent numbers of chromosomes, there must be a genetic
mechanism whereby female development is initiated in diploids and
male development in haploids. h e mechanism involves a single gene
called the complementary sex-determining
(csd)
gene. It is also called
the X locus and the sex locus. Dif erent forms of this gene are called sex
alleles.
4.2.1 Complementary Sex Determination
In 1845, Johannes Dzierson, a parish priest in Silesia, now a province of
Poland, i rst proposed that drone honey bees have mothers but no fa-
thers. h is was the i rst proposal for a sex-determination mechanism for
any animal. However, the complementary sex-determining genic mecha-
nism was not proposed for honey bees until 100 years later by Otto
Mackensen following its discovery by Phineas Westcott (P. W.) Whiting
in
Bracon hebetor,
a small parasitic wasp. Whiting showed that to be a
