Tuesday, February 28, 2006

Wolbachia 101

For the past year, I've been studying associations between insects and Wolbachia, symbiotic bacteria which live inside the cells of arthropods and nematodes. The topic is huge in both depth and breadth, but I'll attempt a brief and simple overview.

Wolbachia are vertically transmitted, which means they pass from parent to offspring. Specifically, they are normally found within the cells of insect reproductive organs, and pass from mother to offspring through the cytoplasm of egg cells; sperm cells, which have little cytoplasm, rarely if ever transmit the bacteria. The association between Wolbachia and insect cells is an obligate one; these are not bacteria that you can streak out on an agar plate.

This tight association means that it's to Wolbachia's evolutionary advantage to manipulate the reproduction of the host, so as to make more daughters that will pass Wolbachia along through the next generation. There are four main ways in which Wolbachia does this.

One way is through the Wolbachia F phenotype, which stands for feminization of genetic males. This phenotype is known in isopod crustaceans (common "pillbugs" and "sowbugs", which are not insects). These arthropods normally have what is called a "ZW" sex determination system. ZW is kind of like our XY system, only backwards; females are "heterogametic", having one Z and one W chromosome, while males are "homogametic" and have two Zs. Wolbachia-infected individuals develop as females, even if they have what would normally be the male ZZ genotype.

Another phenotype is parthenogenesis induction, or PI. This phenotype is best known in parasitoid wasps, tiny insects that lay eggs in the bodies of other arthropods. In infected mothers, the genetic material of the gamete (egg) is duplicated instead of requiring fertilization, and these eggs give rise to diploid daughters. In some of these species, males were unknown until researchers "cured' the Wolbachia infection by giving antibiotics to females. The females then produced unfertilized eggs that developed into haploid males.

A third phenotype, male-killing (MK) does exactly what it sounds like; infected male embryos die early in development. This situation becomes complicated, because it doesn't imply parthenogenesis. Infected mothers put all of their resources into daughter production, and if males can mate more than once, making lots of daughters ensures lots of grandchildren (and more Wolbachia). However, if MK infections become highly prevalent, mates for all these females become scarce, which is favorable to uninfected mothers because sons now become more valuable. For obvious reasons, MK infections don't usually sweep through a population. Under some circumstances, the infection can reach an equilibrium frequency in an arthropod population.

The fourth phenotype is cytoplasmic incompatibility, or CI. The briefest way to describe CI: An infected female can mate successfully with any male, but an uninfected female can reproduce successfully only with an uninfected male. In CI infections, Wolbachia in a male apparently modifies sperm so that it requires "rescue" by the bacteria in an infected egg. Match this altered sperm with an uninfected egg, and the fertilization fails. The result: Infected females have broader mating opportunities than uninfected females, and tend to leave more offspring. Again, a win for Wolbachia, which hitchhike along with the infected daughters.

The molecular and cellular mechanisms by which Wolbachia does its stuff are active research topics. So are its ecological dynamics and consequences, since it can shift sex ratios in an insect population, contribute to the split of one population into two reproductive isolates, or even change phenotype (for example, from CI to MK) in the process of infecting new host species (which it sometimes does, even though it's usually only vertically transmitted). There is considerable genetic diversity among Wolbachia strains, though, and in some cases multiple bacterial strains are found in the same insect species, or even in the same individual.

So, although I'm not a microbiologist (nor do I play one on TV), I spend a lot of time in the lab testing insects for an infection that requires a better microscopist than myself to actually see. Fortunately, we fanciers of multi-cellular eukaryotes can use the polymerase chain reaction (PCR) to amplify Wolbachia genes from preparations of ground-up, infected insects. By current estimates, around 20% of all insect species may be infected, although some estimates are considerably higher, especially since some species may have rare infections that don't show up in the samples collected for PCR screening.

Wolbachia will undoubtedly pop up again in these pages. And, so will the sex determination systems of wasps, ants, and bees. ("Wait a minute ... haploid males? How did they do that? ")

Watch this space!

Friday, February 17, 2006

Some of my favorite sites

Since this blog is so new, I'm still mulling over how to build it up in the future. I suspect its scope will widen considerably, since neither nature nor natural science is easy to partition into neat categories. In the meantime, I thought I'd continue to warm up by posting a few links to other, more established entomologically-oriented sites.

Myrmecos is an insect taxonomist, and an expert photographer of arthropods. His nom de net is derived from an ancient Greek word meaning "ant", and ants are indeed the main focus of his site. There's quite a variety of arthropod imagery there, though; it's well worth a visit if you're a mini-beast fancier.

BugGuide.Net is an online community of amateur and professional entomologists. Any registered user (it's free) can submit photographs, and photographs are searchable. It's a great place to get that strange bug identified, or just to browse the photos and learn more about the taxonomy and natural history of insects.

BugBios, based in Hawaii, is an educational site that includes information on basic insect identification to cultural entomology.

Monarch Watch is an education, conservation, and research site hosted at the University of Kansas. Among its links are resources for teachers, butterfly gardeners, and anyone interested in volunteering to help with field research on U.S. monarch populations.

If you have other favorites, by all means leave a comment. At some point, I'll reconstruct the sidebar to include more links, which I'll probably organize either taxonomically or geographically.

Monday, February 13, 2006

... and a lovesick blues for St. Valentine's Eve ...

This plaintive ditty was first sung to me in August 2000 by a queen ant who had found (and then lost) the love of her life at this event. It's got all the elements of a good blues: Love gone wrong, booze, regrets, baseball, post-nuptial flight de-alation, claustral nest founding, and lots of lines with way too many syllables crammed into 'em. Warning: I've been known to perform it in public given a working keyboard instrument and a suitably bug-crazy audience.


I woke up this morning
Had a sudden urge to fly
I woke up this morning
Had a sudden urge to fly
Better watch out for that windshield
Or I'll be fixin' to go SPLAT! and die.

I got me a drone
The kind you'd follow anywhere
I got me a drone, baby
Yeah, I'd follow him anywhere.
He took me out to the ballgame
Everybody 'round us stop and stare ....

They were runnin' round in circles
And headin' for the door
I said, what's the matter with ya, two eyes?
Ain'tcha never seen no flying ants before?
They don't understand me
How can people be so mean?
They give me the blues so bad
Flying ant blues fit for a queen.

My drone done left me
Mama said it's for the best
My drone done left me
My sisters say he's just like all the rest.
I'm pullin' off my wings now
Ain't never gonna leave my nest.

Sitting here in my brood chamber
Looking at these same old walls
I miss the umpire calling
Miss them walks and strikes and balls
Nobody understands me
Strangest thing these compound eyes have ever seen.
They give me the blues so bad
Flying ant blues fit for a queen.

Gonna call all my workers
They can feed me and make my bed
Gonna say, come to mama, all you workers
Gonna make 'em feed me and make my bed
Gonna tell 'em, forget that melted ice cream ....
Gonna have them bring me some straight whiskey instead.

(Copyright 2000, 2006 by Julie and the anonymous matriarch of a formicid family)

Sunday, February 12, 2006

Happy Darwin Day!

Today, 12 February 2006, is the 197th anniversary of the birth of Charles Darwin. (Yup, he was born on the same day as Abraham Lincoln, a piece of trivia well-worn by yours truly's fellow quiz bowlers.)

Tributes to Darwin and his contributions to modern science have been popping up all over the web, but my favorite so far is Ancestral Magnitudes, written by a group of contributors to Daily Kos. It's a fun read, it's not highly technical, and it does a great job of capturing the awe of the natural world that draws so many of us into science. No explicit bug content, but it does illustrate the fascinating similarities among us siblings-under-the-cuticle.

Saturday, February 11, 2006

The lazy entomologist

From time to time, I'll post a link here to a great bug story instead of trying to write one myself. This wonderfully grisly tale, as told by Carl Zimmer, is one of them.

Thursday, February 09, 2006

The amazing polyphenic Manduca

(Note: Many thanks to PZ Myers, whose own blog gave me the heads up on this one.)

Yuichiro Suzuki and H. Frederik Nijhout have just published the results of a very interesting study of evolutionary change observed in the laboratory. If you have an institutional subscription to Science, you can read it here. Since non-biologists might find the article extremely technical, I'll try to summarize it very briefly:

Every organism has a genotype, which is its (usually) unique complement of genetic material, and a phenotype, which is the way that its genotype is expressed in physical traits. Sometimes we use the terms "genotype" and "phenotype" to describe characteristics of the whole organism or of the interaction of genes; sometimes we use those terms to describe the structure and effect of a single gene. As an example: I have type A blood. Because my mother also has type A blood but my father had type O, I could have inherited the allele for the A antigen only from my mother, and had to have inherited the O allele from my father. So, my genotype with regard to AB blood groups has to be AO. But, because my A allele allows me to make the A antigen, a quick blood test (ouch!) will reveal that my blood phenotype is A.

A polyphenism is a slightly more complicated situation. Most of our visible, developmental, or biochemical traits depend not on single genes, but on interactions of multiple genes with one another -- and often with the environment. In many of the Lepidoptera (butterflies and moths), caterpillars or pupae in a summer brood will have different colors from those in an overwintering brood. One example is the tomato hornworm, Manduca quinquemaculata. M. quinquemaculata caterpillars that develop at 20ºC are black; those that develop at 28ºC are green. This color difference doesn't depend on a genetic difference. A black caterpillar can mature into a moth whose own offspring develop in a warmer seasonal environment and will be green. This is a true polyphenism.

M. quinquemaculata has a close relative, the tobacco hornworm M. sexta. M. sexta has natural color differences as well, but in this case, the differences seen in nature are genetic, and not a polyphenism. M. sexta caterpillars will be green regardless of developmental temperature -- unless they carry only a recessive allele called black, which makes them -- you guessed it -- black. This genetic mutation alters the level of a hormone which regulates, among other things, how pigmentation develops.

However, just as there's plenty of other variation among people who happen to have type A blood, there's other genetic variation among M. sexta individuals that happen to share a larval color phenotype. Reasoning that M. sexta might share some underlying genetic similarity with its close relative M. quinquemaculata, Suzuki and Nijhout set out to expose this variation by exposing black M. sexta to heat shocks more extreme than it would normally encounter in nature. When they applied temperatures of up to 42ºC to developing caterpillars, they discovered that some of the caterpillars would develop green coloration instead, while others changed much less or not at all. So, although it wasn't obvious under natural conditions, there was still variation among black-mutant individuals in how they responded to heat shock. (The color of the green "wild type" caterpillars was not sensitive to heat shock.)

Suzuki and Nijhout then carried their experiment further. They selectively bred a line of moths whose caterpillars were strong responders to heat shock, and a line that didn't respond to heat shock. (Like all good experiments, it had a control -- a line that was heat-shocked but not selectively bred to individuals that responded the same way.) After 13 generations, Suzuki and Nijhout had produced one line of M. sexta that responded to developmental temperature differences with a color change, much like M. quinquemaculata -- and another line that was always black, regardless of heat exposure.

In short, in M. sexta, the black genotype was required for temperature response -- but whether or not a temperature response could be induced in the black caterpillars depended on other genetic variation that was already present in the population. The more technical published details of this experiment describe the physiology of the hormone response, and some other experiments that the authors did to confirm their results. But, my favorite part of the story was revealed by a quick search of the web site of Duke University, where the team did its research. Prof. Nijhout is a well-known researcher in the field of insect developmental biology, but Yuichiro Suzuki, the first author of the paper, is a graduate student. There can't be many things more exciting to a young researcher than a first-authored paper in Science. As Alice learned in Wonderland, it can take real patience to pry answers out of a caterpillar.

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