Biology Reference
In-Depth Information
10.12 Paternal Sex-Ratio Chromosomes and Cytoplasmic
Incompatibility in
Nasonia
Sex ratio in the parasitoid
Nasonia vitripennis
can be altered by at least two dif-
ferent mechanisms. Some natural populations of
N. vitripennis
carry a supernu-
merary or
B chromosome
that causes a condition called
paternal sex ratio
(
PSR
).
B chromosomes are found in many plant and animal species and are small non-
vital chromosomes mostly consisting of heterochromatin. B chromosomes have
few genes and often cause a small fitness cost to their host, making them “self-
ish” genetic elements. Some B chromosomes are thought to be derived from
normal chromosomes and may be transmitted at higher rates than expected,
exhibiting “drive” (
Jones and Rees 1982
).
The PSR chromosome is carried only by male
N. vitripennis
and is transmitted
via sperm to fertilized eggs. After an egg is fertilized by a PSR-bearing sperm,
the paternally derived chromosomes condense into a chromatin mass and are
lost, leaving only the maternal chromosomes. The PSR chromosome itself persists,
changing fertilized diploid (female) eggs into haploid PSR males. PSR is unusual in
its ability to destroy the genome of its carrier each generation (
Werren et al. 1987;
Nur et al. 1988; Beukeboom and Werren 1992, 1993; Beukeboom et al. 1992; Reed
and Werren 1995
).
Where did the PSR chromosome come from? The PSR chromosome has
sequences that are homologous with autosomal sequences of
Nasonia
giraulti
,
N. longicornis
, and
Trichomalopsis dubius
, but not with
N. vitripennis
(
Eickbush et al. 1992
). The PSR chromosome could have been present before
the divergence of the genera
Trichomalopsis
and
Nasonia
(
Eickbush et al. 1992
).
Alternatively, PSR may have crossed the species barrier more recently (horizontal
transfer) through a series of interspecific transfers between species capable of
mating (
Dobson and Tanouye 1998b
). Experimental interspecific transfer of the
PSR was successful after these species were cured of
Wolbachia
, which causes
cytoplasmic incompatibility between them (
Dobson and Tanouye 1998b
). The
transferred PSR chromosome functioned in both recipient species.
The sex ratio of
N. vitripennis
is modified by other non-Mendelian factors
including “Son-killer,” a maternally transmitted bacterium that prevents devel-
opment of unfertilized male eggs (
Huger et al. 1985
) and “Maternal Sex Ratio,”
a cytoplasmically inherited agent that causes female wasps to produce nearly
100% daughters.
10.13 Male Killing in the Coccinellidae
Male killing is associated with a variety of microorganisms. The Coccinellidae
