Biology Reference
In-Depth Information
10.9 Hybrid Sterility
When different species are crossed, hybrid progeny sometimes are produced.
However, the progeny may have altered sex ratios, with one sex absent, rare, or
sterile. The missing or sterile sex is usually the heterogametic sex (
Laurie 1997
).
This phenomenon is known as
Haldane's Rule
, which is “one of a few general
rules in evolutionary biology” (
Sawamura 1996
). However,
Sawamura (1996)
indicates some exceptions do exist to Haldane's rule, caused by maternal effects.
Hurst and Pomiankowski (1991)
suggest that Haldane's Rule only occurs in taxa
with sex chromosome-based meiotic drive, such as the Lepidoptera and Diptera.
Thus, Haldane's rule may be accounted for in some insects by a loss of suppres-
sion of sex-ratio distorters when in the novel nuclear cytotype of the hybrid.
Sex-ratio distorters that result in unisexual sterility in crosses between dif-
ferent species have been found in many species of
Drosophila
, the dipterans
Musca domestica
and
Glossina morsitans
, the hemipteran
Tetraneura ulmi
, and
Lepidoptera (
Acraea encedon
,
Maniola jurtina
,
Danaus chrysippus
,
Philudoria
potatoria
,
Mylothris spica
,
Abraxus grossulariata
, and
Talaeporia tubulosa
)
(
Hurst and Pomiankowski 1991
).
10.10
Medea
in
Tribolium
A class of selfish genes,
Medea
, was found in the red flour beetle,
Tribolium
castaneum
(
Beeman et al. 1992
).
Medea
causes a “
M
aternal-
E
ffect
D
ominant
E
mbryonic
A
rrest” that results in the death of zygotes that do not carry it. If a
mother carries
Medea
, any of her offspring that lack this gene die before they
pupate. Females who are heterozygous for
Medea
lose half their progeny if
they mate with a wild-type male and ¼ of their progeny when mated to a het-
erozygous male. It was hypothesized that
Medea
could lead to reproductive iso-
lation and speciation in
T. castaneum
populations.
A survey of wild populations of
T. castaneum
from Europe, North and South
America, Africa, and Southeast Asia showed that four different
Medea
alleles
were widespread, but absent or rare in Australia and the Indian subcontinent
(
Beeman and Friesen 1999
).
Thomson and Beeman (1999)
suggest that
Medea
factors are absent from India because a hybrid incompatibility factor (H) is
found in the
T. castaneum
populations in India. Apparently, H and
Medea
strains
of
T. castaneum
are incompatible due to suppression by the H factor of the self-
rescuing activity of the lethal
Medea
genes.
10.11 Cytoplasmic Agents Distort Normal Sex Ratios
Many cytoplasmically transmitted organisms (bacteria, viruses, and protozoa)
alter the “normal” sex-determining mechanism(s) in arthropods (
Table 10.3
).
Most are inherited primarily through the oocyte of the mother (cytoplasmically