IGERT2009: Sunday morning session

My little laptop is functional again, so at least I'll be able to blog these Sunday morning IGERT sessions in real-time. I still have to transcribe my notes from yesterday; I'll plan on getting that done on the plane this afternoon.

Kristi Montooth: Mitochondrial-nuclear epistasis for metabolic fitness in flies

How do physiological systems evolve to maintain metabolic fitness? This is a process that involves interactions between two genomes, the nuclear and mitochondrial. Energy metabolism is important and is the target of mutation, but the same players are found all across the tree of life, suggesting that there is also strong selective pressure to maintain a common system.

Montooth is looking at inducible gene expression: is there an energetic cost to switch genes off and on? She's using respirometers that can measure the metabolic rate of single flies or larva. Flies are subjected to heat shock, which switches on HSP70. Flies normally have 6 copies of HSP70; they have mutants with 12, and they show a much greater rise in metabolic rate in response to heat shock.

Mitochondria are the source of the energy for this response. Mitochondria also have a high mutation rate and show strong linkage (no sexual recombination to cover for errors that arise). She's arguing for selection for compensatory evolution in the nuclear genome, and the accumulation of intergenomic epistasis. To dissect the effects of coevolution of mitochondria and nuclear genomes, she transplanted mitochondria from different species into Drosophila melanogaster. These have between 18 and 100 amino acid substitutions from the Dmel sequence.

She plots mitochondrial genome in order of increasing divergence against measured fitness (she used a competition assay that she did not describe in detail). There is no correlation seen at all. Also, high fitness X/mtDNA genotypes in one sex can be low fitness genotypes in the other sex. Interactions between the X and mtDNA can maintain variation in both genomes. All of the fitness effects, with one exception, are subtle.

Some of the transgenomic effects have very strong effects on female fecundity, developmental rates, and locomotion. But adult metabolic rate shows no difference! The idea is that there are lots of homeostatic mechanisms that maintain metabolism very tightly, which then have secondary effects.

Johanna Schmitt: Adaptive evolution of Arabidopsis flowering pathways in different climates

Schmitt does ecological development, looking at the timing of plant development in different environments. How does phenology respond and adapt to climate variation? We expect evolution to adapt to variation in seasonal timing. The signaling pathways in Arabidopsis are well known; they respond to hormones, photoperiod, and ambient temperature by way of a fairly complicated set of pathways she showed us in a slide...sorry, no way I can reproduce it here!

Across its range, it shows a great deal of life history variation; one pattern in the Mediterranean, another in colder northern climes, and yet another in Northern Scandinavia, varying in how much time they spend in vegetative rosettes vs. bolting and flower production. Questions: are there are genetic variants associated with different life history patterns, can they identify the genes, and can they perturb them?

The experiments involved massive plantings in different sites in Europe with different climates, with different mutants. Is natural variation in candidate genes involved in variation in flowering time? They studied FRIGIDA, a gene that effects the vernalization pathway. When you lose FRIGIDA, you should see much more rapid flowering. Loss of function in this gene has evolved multiple times in northwestern Europe. The effect depends on the timing of planting and climate.

The effect of the mutant varies across geography, and they have a photothermal model of flowering time. The plants are tracking light and temperature, and the different mutants are counting up these inputs in slightly different ways. They can use this model to make predictions on the effects of FRIGIDA on flowering time with changes in germination timing, and then test these in the next year with plantings at different times and in their different geographical sites, and the model is working accurately.

They are also plugging in predicted future climate change from NOAA, and asking what we can expect to see 100 years from now; she showed maps of expected flowering times in 2100. They are also making predictions of the expected distributions of FRIGIDA alleles over time, and they hope to do the same for many other alleles in Arabidopsis.

Artyom Kopp: How the fly got its sexy legs - the origin and evolution of Drosophila sex combs

The sex comb is a male specific structure on the front legs which most Drosophila species lack -- it's a fairly recent innovation. How do you evolve a novel structure?

It's limited to the melanogaster and obscura species groups, with quite a bit of diversity in different species, varying from 2-50 teeth, location, and arrangement. How do you go from sexually monomorphic state of a generically hairy leg to one with a specific bristle arrangement in males? The sex comb in males is homologous to a subset of bristles also found in females; in males, that patch of epidermis rotates 90° and the bristles enlarge. He showed a very pretty developmental series of this epithelium undergoing cell shape changes that move the bristles to a new location. Other species show similar morphological remodeling, but sometimes with some significant differences: D. kikkawai doesn't do the rotation, but instead the bristle precursors arise in their final position. These modes do not cluster together phylogenetically, so these are examples of convergent evolution, generating similar structures with different mechanisms.

They are taking apart the genetics and regulatory inputs of sex comb development. Basically, it involves just about everything. It seems to arise by an interaction between Hox and sex determination genes. Spatial modulation of Sex combs reduced controls sex comb position. Scr in pupa; stages is only expressed in a limited domain in the leg, and ectopic expression of Scr produces multiple sex combs. Expression is also sexually dimorphic, with no upregulation of Scr in female legs. In D. ficusphila, which has enormous sex combs, Scr levels are elevated yet further to 7 times the levels found in D. willistoni.

The sex determination gene Double sex is also spatially patterned, and is refined and elevated to high levels in the area around the developing sex combs. Ectopic expression of Dsx induces ectopic sex combs.

How can a new developmental pathway evolve? In the ancestral condition, Scr is controlled by spatial cues to produce segmental patterns of bristles; in the sex-comb carrying species, Scr is coupled to Dsx. This explains the spatial pattern of gene expression, but it also needs to acquire new downstream targets to, for instance, regulate epidermal rotations.

Drosophila are old, and many of these species differences are millions of years old. They are now looking at more recently diverged species with differences in sex comb morphology, and are looking for correlations between Scr and species divergence.

And with that, I have to run for the airport shuttle. Good talks, and I unfortunately have to miss Rudy Raff's wrap-up of the meeting.

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