MrDarwin

In today's Washington Post:

Walking Sticks, Just Winging It

Interesting stuff, but something I discussed with my graduate advisor almost 15 years ago when I was doing a cladistic analysis of a plant group. To be honest, it's not that terribly surprising that the genes for a complex trait can be "turned off" for some number of generations, then "turned on" again later. You don't have to lose the genes, only the genetic signal to activate them; in plants it's inescapable that complex characters have come and gone repeatedly.

One thing I do take issue with is the article's suggestion that these genes may have lain dormant, and more or less intact, for 50 million years. I'd like to see the original article, and see if that's what the authors actually say. I'd also like to find our their reason for thinking that wings were not lost multiple times in different lineages, i.e., a winged clade that is well-nested in a wingless clade may have regained wings--the parsimonious answer--or the other members of the clade may all have lost wings independently, a less parsimonious answer but we already know evolution is not always parsimonious!

Outblaze

So where does Dollo's Law (basically stating that organisms cannot re-evolve along lost pathways, but must find alternate routes) come in to play concerning the idea of switching regulatory genes on and off? Or does it?

Nic Tamzek

Nature's website appears to be jacked at the moment.

However, I also think that multiple-loss is perhaps a more likely explanation, not knowing anything in particular about the details of the case.

Myrmecos of ARN and other places is a bug expert, I'm sure he'd have something intelligent to say...

Emma Peel

Here's the New Scientist article. It includes a phylogenetic tree. As I read it, wingless walking sticks split off from winged web spinners. Then some walking sticks regained wings, and some of their descendent species lost them again.

BYU NewsNet ran an article that concentrated on the fact that one of the authors of the study was a lowly BYU undergrad.

Nic Tamzek

Courtesy of New Scientist:

[well, durnit, it looks like this img won't work, go to the article]




Looks to me like we could hypothesize (at the extremes):

(a) Wingless walking stick ancestor and re-evolution of wings 4 independent times

(b) Loss of wings 6 times from a winged ancestor.

In terms of raw numbers, (a) is more parsimonious, but it wouldn't take much of a probability difference *at all* to make (b) more parsimonious. E.g., if say the loss of wings is twice as "easy" as the reinvention of them (and this is almost certainly a vast underestimate).

I take it that all these wings are developmentally similar, etc. -- if they were like, say, fish fins, dolphin flippers and penguin flippers then the case for re-invention would be much stronger, but the authors wouldn't (accurately) be discussing Dollo's Law.

So, without having read the paper, I vote b!! Screw Nature and the science media!!



nic

MrDarwin

It means Dollo's Law is pretty much out the window.

Looking at the tree, I can't figure out the justification for the common ancestor of all walking sticks being wingless. Since the sister group (webspinners) is winged, and one of the basal branches is winged, the suggests to me that the ancestor of all walking sticks was also winged. That still leaves some problems with the other clade, but I'm wondering if there might be some strong selective pressure for walking sticks to lose wings.

I'm thinking of a molecular analysis of a plant group I work on in which we have sampled a large number of genera and species, with excellent support for berry fruit having evolved independently from capsular fruit (the unambiguous ancestral state) at least twice. But if we only knew about those two berried taxa and one or two capsuled taxa (i.e., enough members of this clade were extinct or unknown), the parsimonious solution would be a single origin of berries from capsules, and a capsuled sister clade of a berry taxon (as shown by the molecular analysis) would then appear to have "reverted" to a capsular fruit, i.e., it would appear to be nested within a berried clade.

(But I should also add that I've always been rather skeptical of "Dollo's Law". We know enough about genetics now to know that genes can be retained, sometimes for millions of years, without being expressed; if a mutation in a regulatory gene is "fixed" by a subsequent mutation then you can revert to an ancestral character state in a single generation; atavistic hairiness in the Mexican "wolf boys" might be one such example in humans).

RufusAtticus

Okay, let's hope this works. . . .

The suplementary information is really all anyone needs to look at. It shows the phylogeny where wings are lost twice and regained five times.

MrDarwin

Okay, that makes more sense and is considerably more convincing. It shows numerous basal wingless clades that were omitted from the other, apparently oversimplified, diagram.

But what needs to be emphasized is that this kind of analysis can only provide morphological character states for the living representatives of the group; character states are assumed or extrapolated for the ancestors.

What they need to do now is look at the fossil record for the group, and see if all the early fossil representatives are also wingless--this would go a long way towards supporting their hypothesis. But I'm not sure how well this group is represented in the fossil record.

RufusAtticus

Exactly what I was thinking! How much you want to bet that there is already a paper in the works?

Albion

The original article's in this week's Nature:

http://www.nature.com/nature/

Here's an extract that I hope is short enough to not be a copyright problem:

"Entomologists have long assumed that re-evolution of wings in apterous lineages was impossible, because functional wings require complex interactions among multiple structures, and the associated genes would be free to accumulate mutations in wingless lineages, effectively blocking the path for any future wing reacquisition. However, this assumption requires that developmental pathways for wing formation are largely independent of pathways required for development of other structures. For instance, in Drosophila and other insects, leg and wing imaginal discs have a common origin from a single group of cells and the developmental pathway for wing determination has been largely co-opted (recruited) from the pathway required for limb formation15, 16. Therefore it is not surprising that the basic genetic instructions for wing formation are conserved in wingless insects, because similar instructions are required to form legs, and probably other critical structures16. Studies of flight motor patterns in flying and non-flying phasmids indicate that the non-flying phasmids have retained the neural structures and basic functional circuitry required for flight, as indicated by flight-specific neural activity in thoracic muscles17, demonstrating that the loss of wings does not correlate with the loss of flight musculature and innervation. Wing development depends on multiple gene systems, transcription factors, secreted proteins, and receptors15, and mutations in any one of these factors may lead to winglessness. Given the multitude of factors involved in wing formation, it seems probable that the specific cause for winglessness will differ from lineage to lineage, but that the basic blueprint for wing formation can remain largely intact, even over large evolutionary time periods."