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Poster Full Abstracts - Evolution and Quantitative Genetics
Poster board number is above title. The first author is the presenter
257
Because two out of every three X chromosomes in a population with 1:1 sex ratio are found in females, sex-specific selection pressures are expected to
differentially affect X-linked and autosomal genes. Numerous studies have shown that genes with male- (female-) biased expression are under- (over-)
represented on the
Drosophila
X chromosome, but the specific factors responsible are not known. Notably, genes with male-biased expression tend to be
narrowly expressed in a limited number of tissues, and narrowly expressed genes are also under-represented on the
Drosophila
X chromosome. It is
therefore unclear whether the non-random X-linkage of genes with sex-biased expression is the result of selection on genes with sex-biased expression,
narrowly expressed genes, or some combination of the two. To address this problem, we measured sex-biased expression in multiple
Drosophila
species in
head and whole fly and at different developmental time-points, and we obtained expression measurements from 14 adult tissues. Our results suggest that the
paucity of X-linked genes with male-biased expression is driven primarily by a dearth of genes with accessory-gland-biased expression on the X
chromosomes, and this dearth is a product of both male-specific selection pressures being weaker on X-linked genes and limits imposed by dosage
compensation. In addition, there is also an excess of X-linked genes with female-biased expression, even after controlling for the confounding factor of
expression breadth, suggesting that the female-biased transmission of the X chromosome favors the accumulation of female-beneficial substitutions in X-
linked genes. These results demonstrate that the unique gene content of the X chromosome is a product of sex-specific selection pressures, and not an
outcome of selection on narrowly expressed genes. Lastly, the expression levels of X-linked genes diverge faster between species than autosomal genes,
suggesting that genes on the X chromosome are particularly evolutionarily labile, which may promote the unique gene content of this chromosome.
466A
Expression pattern evolution of three parent genes and their retrogene copies in
Drosophila
species.
Ryan S. O'Neill, Denise V. Clark. Biology Dept,
University of New Brunswick, Fredericton, NB, Canada.
Following gene duplication, gene copies may become specialized for different parts of the ancestral single copy function (subfunctionalization), or they
may attain new functions (neofunctionalization). Retrogenes are gene copies that arise when processed mRNAs are reverse transcribed and inserted in the
genome. While new genes generated through DNA-based gene duplication can retain the regulatory elements and introns of their parent gene, retrogenes
lack these elements. A new retrogene is therefore unlikely to share its parent gene's expression pattern. In
Drosophila melanogaster
, retrogenes generally
have narrower expression patterns than their parent genes, but, without knowing the ancestral pattern, we cannot determine whether the retrogene’s
expression is the result of subfunctionalization or neofunctionalization. In this study, the expression patterns of three parent genes and their retrogene copies
are examined in several
Drosophila
species using
in situ
hybridization. In
D. melanogaster
, these parent genes and their retrogenes are
CG17734
and
CG11825
,
CG8331
and
CG4960
, and
Sep2
and
Sep5
. The genome sequences of species in the
Drosophilidae
lineage show that the three retrogenes arose
within this lineage, while the parent genes have persisted in all
Drosophila
species. The parent gene
CG17734
is expressed during fewer embryonic stages in
species with the retrogene compared to species without the retrogene, whereas the retrogene
CG11825
is expressed in those stages where
CG17734
expression is lacking, indicating that this gene pair may have evolved via subfunctionalization. For the other gene pairs, the orthologous parent genes have
conserved expression patterns across species, regardless of whether or not a retrogene copy is present, suggesting that these parent genes have maintained
their ancestral functions and have not undergone subfunctionalization following the arrival of the retrogene. However, retrogenes
CG4960
and
Sep5
have
variable expression patterns across species, indicating prolonged functional diversification of these retrogenes.
467B
The molecular basis of speciation in Drosophila.
Nitin Phadnis, Harmit Malik. Division of Basic Science, Fred Hutchinson Cancer Research Center,
Seattle, WA.
Speciation, the process by which one species splits into two, involves the evolution of reproductive isolation between previously interbreeding populations.
A central goal in evolutionary biology is to identify the genes and the evolutionary forces that cause reproductive isolation. The idea that genetic conflict
involving segregation distorters may drive of the rapid evolution of genes underlying reproductive isolation is intuitively appealing, but empirical evidence is
limited. Previously, we showed that a single gene
Overdrive
causes both male sterility and segregation distortion in
Drosophila pseudoobscura
Bogota-USA
hybrids. Here, we perform a genome-wide genetic dissection to provide a comprehensive look at the genetic architecture of all components of the hybrid
incompatibility underlying F1 hybrid sterility. Postzygotic isolation between Bogota and USA involves a single incompatibility consisting of only a handful
large effect factors. The genetic bases of hybrid sterility and segregation distortion are largely but not completely overlapping. Identification and
characterization of these genes, including
Overdrive
, are providing important insights into the molecular nature of reproductive barriers that isolate species.
468C
A possible contribution of
abrupt
in the evolution of beetle elytra.
Padmapriyadarshini Ravisankar, Nagraj Sambrani, Yoshinori Tomoyasu. Zoology
Department, Miami University, Oxford, OH.
Morphological innovation is a fundamental process in evolution, yet the molecular mechanism underlying the evolution of morphologically novel
structures is still elusive. Coleoptera (beetles) is the most successful animal group on the planet, accounting for over 20 percent of extant animals. Innovation
of elytra, which are highly sclerotized and modified forewings, is an important trait driving the successful radiation of beetles. We are using the red flour
beetle,
Tribolium castaneum
, as a model system to understand the molecular basis of elytral evolution in beetles.
Tribolium
is rapidly gaining momentum as
a genetic model system due to its availability of several modern genetic tools. Systemic RNAi technique is one of the important advantages in
Tribolium
,
which has paved the way to create gene 'knock down' phenotypes by simple injection of double stranded RNA (dsRNA). Our initial RNA interference
(RNAi) screening for genes important for the evolution of elytra has identified
abrupt
(
ab
) as a gene involved in the formation of unique elytral features.
ab
encodes an evolutionary conserved transcription factor that contains a BTB zinc finger domain. A mutation in
ab
in the fruit fly
Drosophila
results in the
loss of a particular wing vein. Depleting
ab
function via RNAi in
Tribolium
also caused defects in some wing veins, suggesting that the function of
ab
in the
wing vein formation is conserved among insects. In addition, we noticed that
ab
is essential for elytron and hindwing cell proliferation in the early dorsal
appendage development in
Tribolium
. RNAi analysis in
Drosophila
has revealed that this novel
ab
function is indeed conserved even in
Drosophila
.
Interestingly,
ab
RNAi also affected the formation of several features only seen in elytra. For example, the unique overall shape of elytra was altered by
ab
RNAi. This suggests that, in addition to several conserved functions,
ab
has gained a new function in the beetle lineage, which might have contributed to the
elytral evolution.