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