Developmental homology and dissociation Homologous genes need not function in the development of...
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Developmental homology and dissociation
Homologous genes need not function in the development of homologous structures(HOX genes, Notch signaling)
Expression of a homologous gene does not imply that developmental pathways are also homologous(engrailed and metamerism)
Homologous developmental pathways may control the development of non-homologous structures(Dll in appendages, Pax6 in the eyes)
Homologous structures need not be specified by homologous genes (insect segmentation)
Segmentation of the Drosophila embryo
Genetic control of segmentation in Drosophila
Segment polarity genes make up the bottom level of the regulatory hierarchy
Segment polarity genes establish boundaries between segments and control patterning within each segment
Expression of segment polarity genes is conserved in beetles…
en
wg
Tribolium
Grasshoppers
en
Schistocerca
Myriapods
en/wg
Lithobius
Crustaceans
wgArtemia
Spiders
en
wg
Cupiennius salei
The functions of en and wg in subdividing the embryo into segments appear to be conserved
en
krusty, a gap mutants in Tribolium
Pair-rule mutants in Tribolium
eve pair-rule function is conserved in beetles
Chromophore-assisted laser inactivation
Antennae
ftz deletion does not affect segmentation in Tribolium
Pair-rule gene expression in Schistocerca
eve
ftz
Drosophila Tribolium
pby
even-skipped expression in Lithobius
even-skipped expression in Lithobius
eve expression does not show two-segment periodicity
eve/ en
Expression of pair-rule genes in Chelicerates
primary pair-rule genes in Cupiennius
paired - secondary pair-rule in Tetranychus
Evolution of Arthropod segmentation
- Some parts of the segmentation pathway are conserved. - There is some turnover of genes within the overall pathway
- Pair-rule patterning may be a higher insect innovation
Segmentation in long germ band insects
- Simultaneous generation of segments- Segmentation independent of growth- Occurs in syncytial environment
QuickTime™ and aPlanar RGB decompressorare needed to see this picture.
Cellularization before blastoderm formation in grasshoppers
Rhodamine dextran injection
Segmentation in short germ band insects (Tribolium)
-Sequential generation of segments-Segmentation coupled to growth- Occurs in a cellular environment
The global patterning mechanisms cannot operate in the same way as in Drosophila
Maternal gradient of bicoid establishes Anterior-Posterior axis in the Drosophila embryo
The roles of maternal gradients in Drosophila
Gradients form from maternally deposited transcripts by diffusion or transport in a cell-free environment
bicoid function is conserved in Cyclorhapha
Inhibition of Bcd protein synthesis in Megaselia results in posterior duplication (an embryo with two butts…)
Maternal gradients in Drosophila and Megaselia RNA interference
bicoid does not exist outside higher Dipterans
hunchback is a conserved component of the Anterior determination system
Megaselia
Schistocerca
RNA interference
Tribolium hunchback is correctly regulated in Drosophila
Tribolium hb Tribolium hb transgene in Drosophila
Some maternal system must therefore exist in beetles
But how does it work without bicoid?
hunchback can substitute for bicoid
Making anterior hunchback stripe in the absence of bicoid
In Tribolium, anterior patterning is controlled by orthodenticle and hunchback
otd is deposited maternally
Removal of otd and hb eliminates anterior structures
otd
hb
otd; hb
A new mechanism for a new mode of development
Drosophila Tribolium
Maternal
Zygotic
bcd hb
otd hb
Anterior structures
otd
hb
Anterior structures
A maternal protein gradient can only work in a syncytium
bcd has taken over the ancestral functions of otd and hb ?
A parasitic wasp, Copidosoma floridanum
Polyembryonic development
Polyembryonic development
Primary morula Polymorula Secondary morulae
Polyembryonic development in Copidosoma floridanum
Polyembryonic development evolved independently as an adaptation to parasitism
engrailed expression in Aphidius ervi
Segmentation without maternal gradients
eve eve
en
Segmentation without pair-rule genes?
Bracon
Aphidius
How do developmental pathways diverge from a common ancestral state?
- By recruitment and loss of component genes
- By re-deployment of old genes in new patterns
- By changing regulatory interactions between genes
Somatic sex determination pathway in Drosophila
Sex determination is cell-autonomous (X:A ratio or dsx expression)
Somatic sex in Drosophila is controlled by a splicing cascade
Establishment (X:A ratio)
Maintenance (autoregulation)
Regulation of downstream target genes by doublesex
Genotype/sex Yolk protein expression
Sex determination mechanisms in insects
Y-chromosomal genes (Tipulidae, Tephritidae)
Autosomal genes (Culex, Anopheles)
Mobile genes (Megaselia, Musca)
X:Autosome ratio (Drosophila)
Genotype of the mother (Chrysomia, Sciara)
Haploid/ diploid (Hymenopterans)
Environmental factors (Pseudacteon)
Sex determination in the medfly Ceratitis capitata (Tephritidae)
Sxl
tra
Sex is controlled by a male-determining factor on the Y
Differential splicing of transformer in Ceratitis
transformer controls sexual differentiation in Ceratitis
Female Male
Intersexes produced by tra RNAi
Sxl dsx
Sxl and dsx in Megaselia scalaris (Phoridae)
Sex determination in Megaselia
Megaselia lacks differentiated sex chromosomesThe Maleness factor is mobile and can be located on different chromosomesThis can create new Y chromosomes from former autosomes
Sex determination systems in Musca domestica
In male-heterogametic strains, sex is determined by a single masculinizing factor (M), which can be located either on a Y chromosome, or on 4 different autosomes
Some female-heterogametic strains are homozygous for M, and sex is determined by a dominant feminizing factor F
Other female-heterogametic strains lack M, and sex is determined by a recessive masculinizing factor Fman
In arrhenogenic strains, sex of the offspring depends on the genotype of the mother
Sexually dimorphic splicing of dsx is conserved in Musca
Musca doublesex expression is sexually dimorphic
Somatic sex correlates with sexually dimorphic doublesex splicing, irrespective of the upstream sex determination mechanism
Musca dsxF induces vitellogenin synthesis in males
Musca dsxM promotes male-specific morphology in Drosophila
Musca dsxM has the same phenotypic effect as Drosophila dsxM
The mechanism of sex-specific differentiation appears to be conserved, even if the upstream sex determination signal is not
dsx is required for the development of reproductive organs in Musca
F
dsx
dsxM dsxF
M
A model for Musca sex determination?
Suppose that:
dsx produces a male-specific product by defaultF is required for female-specific splicing of dsxM represses F
Then:
F-D could be an M-insensitive allele of FF-man could be a non-functional allele of FAg and tra could be leaky alleles of F or M expressed
in the germiline??
Could F be a homologue of theDrosophila transformer gene?
Bombyx mori
doublesex controls sexual differentiation in Lepidoptera
Sex determination in Lepidoptera
In Lepidopterans, females are the heterogametic sex (males ZZ, females ZW)
W? or Z:A?
Splicing regulator (tra?)
Differential splicing of dsx
dsxM dsxF
Male development Female development
Evolution of the sex determination pathway
The function of doublesex in sexual differentiation and the sexually dimorphic splicing of dsx are highly conserved
The immediate upstream regulator of dsx splicing (tra) may also be conserved?
The primary sex determination signals evolve rapidly and vary among closely related groups
Presumably, the downstream targets of dsx are also different in different species
dsx
Sxl “measures” the X:A ratio reflected in the balance between “numerator” and “denominator” gene products
Numerator genes are located on the X chromosome (sisA, sisB, sisC, and runt); a single denominator element (deadpan) is located on an autosome
How does the fly count to 2?
Sxl protein is highly conserved in all Dipterans: Musca (83%), Chrysomia, Ceratitis, Megaselia, etc.
Sxl is spliced in a sexually dimorphic fashion in other Drosophila species (D. virilis, D. subobscura), but not in other Dipterans (Musca, Megaselia, Ceratitis).
Dipterans outside Drosophila also lack the male-specific, translation-terminating exon
Although the RNA binding domain of Sxl is highly conserved, Sxl proteins from Musca or Ceratitis cannot regulate the splicing of tra in transgenic Drosophila
Sex determination mechanism based on Sxl is a Drosophila innovation
So where did the numerator elements come from?
Sxl was recently recruited in the sex determination cascade
The origin of numerator genes
The role of Sxl in sex determination is very recent, so the numerator genes must also have acquired their functions in sex determination recently
All numerator genes have other functions in development that clearly predate their roles in sex determination: segmentation (runt), neurogenesis (sc, da, dpn), signaling (upd)
What does it take to be a numerator gene?
Genes must be located on the X chromosome
Must be able to regulate gene expression
Must be expressed very early in development
One of the numerator genes is scute
bHLH transcription factor with a very ancient function as a proneural gene
Part of a gene cluster evolved by tandem duplications (scute, achaete, l(sc))
Shares regulatory elements and overlapping expression with achaete
Changes in scute regulation were responsible for its recruitment in the X:A sex determination signal
The duplication of achaete and scute predates the recruitment of scute as a numerator element
In contrast to neurogenesis, achaete cannot substitute for scute in its sex determination role
Changes in scute coding sequence were not required for the acquisition of its new function
The numerator function of scute depends on specific cis-regulatory elements
Recruitment of scute for the numerator function
Models of pathway evolution
Retrograde growth from a simple ancestral state
Component replacement
Emancipation and regrowth