Biology Reference
In-Depth Information
Furthermore, sex determination in insects appears to be based on alterna-
tive splicing of
tra
+
mRNAs, based on studies in Diptera and the Hymenoptera
(
Salz 2011
).
10.8 Meiotic Drive Can Distort Sex Ratios
Meiotic drive
alters the assortment of chromosomes during meiosis so that cer-
tain chromosomes are inherited more frequently than expected (
>
50%). Meiotic
drive most frequently is observed when sex chromosome allocation is disrupted
(sex-chromosome meiotic drive) so that the sex ratio is altered. Whether meiotic-
drive mechanisms actually modify sex-chromosome distribution more frequently
than autosomal chromosomes is unknown (
Lyttle 1993
).
Sex-chromosome meiotic drive has been found most often in the Diptera, includ-
ing the
Drosophila obscura
,
melanica
,
tripunctata
,
testacea
,
melanogaster
, and
quinaria
groups, mosquitoes (
Aedes
and
Culex
), sciarid flies, and stalk-eyed flies
(Diopsidae) (
Jiggins et al. 1999
). The frequency of meiotic drive in other insects is
unclear (
Jiggins et al. 1999, Jaenike 2001
). Three examples of sex-chromosome mei-
otic drive in Diptera are described in Sections 10.8.1-10.8.3, including Segregation
Distorter (SD) in
Drosophila
, male drive (MD) in the mosquitoes
Aedes aegypti
and
Culex quinquefasciatus
, and meiotic drive in stalk-eyed flies.
10.8.1 Segregation Distorter (SD)
In
Drosophila melanogaster
, the SD phenotype is present at low, but stable,
frequencies in most field populations. Males of
D. melanogaster
that are hetero-
zygous for the (SD) chromosome (SD/SD
+
) may produce only progeny with the
SD chromosome, instead of half with SD and half with SD
+
, due to the failure of
sperm with the SD
+
chromosome to mature (
Ashburner 1989, Ganetzky 2000
).
Segregation distortion occurs because the nuclei of the sperm with the normal
SD
+
chromosome fail to condense normally at sperm maturation. Another gene,
the Enhancer locus of SD,
E
(
SD
), is required for the full expression of meiotic drive.
The SD “locus” actually consists of two overlapping genes, HS2ST and RanGAP.
RanGAP is an essential component of a system that transports proteins and RNA
molecules into and out of the cell's nucleus. Both HS2ST and RanGAP actually
are present twice on the SD chromosome, as opposed to once on SD
+
chro-
mosomes, and the tandem duplication is necessary for segregation distortion
(
Palopoli et al. 1994
). Both genes appear normal in the right-hand copy, but
the RanGAP gene on the left lacks the last 234 amino acids (
Merrill et al. 1999,
Ganetzky 2000
).
