A question of the spotty record

I have spent an inordinate amount of time studying Loxandrine carabid beetles. This group has about 240 described species, all but a couple dozen from the Western Hemisphere. The bulk of the species diversity is in South America where we find nearly 200 species, but by my estimate, this is less than half of the true number. When full described they should top 500 species. A respectably large clade of beetles, if not crazy mega-diverse.

A spotty undescribed Loxandrus from Ecuador.

A spotty undescribed Loxandrus from Ecuador.

One persistent mystery is the distribution of color patterns in the group. Among the South and Central American species (and a few in North America) patterns of pale spots and vittae on the elytra are very common. Typically markings are humeral (near the “shoulder” of the elytra), along the parasutural interval (the first interval in the middle of the elytra) or apical (either as a central spot or along the lateral margin). The markings can be variously shaped and are red or yellow. These sorts of marks and their positions are seen across just about all carabid tribes. So common that one must seriously consider the possibility of a good adaptive reason. Perhaps they are aposomatic (warning would-be predators of the defensive chemicals they have) or the pattern is disruptive, enabling their escape from visual predators by breaking up the beetle-shaped outline.

A spotless Zeodera atra from Australia

A spotless Zeodera atra from Australia

Just jump over to Australia and things look quite different. The diversity is smaller, all the species of loxandrines of Australia, New Guinea and New Caledonia will only amount to about 100 species and most strikingly, not a single spot on any species except for one rather poorly marked, undescribed Cerabilia. Other carabids in Australia seem spotty enough.  For example there are beautifully marked Leconomerus (Harpalini) and Abacetus (Abacetini). The one Cerabilia with it subtle vittae is found with a similarly marked Leconomerus, so perhaps mimicry is involved.

There is a question raised for me. What is it about the evolution and environment that these sister clades (Australian and New World taxa are reciprocally monophyletic) experienced that led to the spotty and spotless species we see now?

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About Kip Will

I'm an insect systematist with expertise in carabid beetles, who is always happiest in the bush.
This entry was posted in Carabids, Coleoptera, Pterostichines. Bookmark the permalink.

8 Responses to A question of the spotty record

  1. You are going to have to learn how to deal with ignorant questoners, like me.

    What’s the problem? The Australian and American groupings are separate sister clades. Spottiness appeared (or disappeared) in the last common ancestor of one lot but not the other. This may have been adaptive, or just drift.

    • Kip Will says:

      Hi, thanks for the comment. I would say there isn’t a problem, really. Rather there is something intriguing about sister lineages where one has a prominent feature that the other lacks. As you say, one condition or the other was inherited from the common ancestor and then gains or losses happened. Thus the pattern is described. What of the process then? It seems that if it was more or less neutrally evolving (no particular selection pressure one way or the other) then given the 80 to 30 million years since their last common ancestor, I would expect to see a more or less random distribution of spotty patterns in both lineages. Lots of spots in one sister and none in the other is about as far from the neutral expectation as we can get. Also color patterns are pretty well documented as being involved in predator-prey interaction (though not in these beetles) so it appears to be a good candidate for that kind of selection. If we had a good phylogeny of the New World loxies it would be a lot easier to draw conclusion. Someday…Someday.

      • Alternatively, there could be a specific difference at a control gene (is that the right term?) that makes all the difference?

      • Kip Will says:

        The two aren’t really alternatives as both do exist. There is certainly a genetic mechanism that causes their colors, either spotty or not. Assuming variation in color happens, and we do see some variability, then something has driven or maintains spots or their lack. The mutation(s) that lead to color patterns, for example, appears to have happened and then gone to fixation many times across the New World taxa. This is suggestive of a selective pressure that they are encountering repeatedly in different lineages that drive convergence. Much the same way that the selective pressures in a cave seem to lead to eye loss repeatedly.

      • “The mutation(s) that lead to color patterns, for example, appears to have happened and then gone to fixation many times across the New World taxa. ” Do we know this? You lament the lack of a good phylogeny. I’m reminded of the stripe problem among equids; IIRC current phylogenies imply gain (or loss) of stripiness more than once.

      • Kip Will says:

        My weasel word is “appears.” I do have a phylogeny and it has at least 3 origins of spots in the New World clade, but it includes only a few dozen (well chosen) of the hundreds of species in South America. I will be surprised if there are less than 3 origins of patterns, but not surprised if they are also repeatedly lost.

  2. James C. Bergdahl says:

    Interesting question; I can see how you cannot avoid seaching for an explanation.

    Of course, many Bembidion also show wide interspecific variation in “spots” and similar markings on their elytra, even with subgenera, which also begs for an explanation.

    You mention: “Perhaps they are aposomatic (warning would-be predators of the defensive chemicals they have).” Many of the species with “spotty” elytra are primarily nocturnal, and therefore one would expect less selection pressure by predators using acute vision. A nocturnal life style seems to strongly select for monochromatic dark body coloration in carabids. Also, many carabids are capable of releasing defense compounds but have no obvious warning coloration. I suppose there may be some variation in the “accessibility” for natural selection of genetic mechanisms affecting elytra spots across carabid taxa, and some loxandrines and bembidines ataxa re predisposed in this regard.

    I am not familiar with loxandrines, but I assume many (most?) of them are primarily nocturnal in all regions they occur in.

    James Bergdahl
    Spokane, WA, USA

    • James C. Bergdahl says:

      Chances are you have read:

      Brown, W. L., Jr. 1985. The Quatrimaculate color pattern in beetles and a possible mimicry complex. Pp. 432-437, In: G. E. Ball (ed.), Taxonomy, phylogeny and zoogeography of beetles and ants…….

      James C. Bergdahl
      Spokane, WA, USA

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