Drosophila forever?

As I've commented before on this blog, taxonomy holds an unusual position in the biological sciences in that it fills two equally significant roles. On the one hand, it is a science in its own right, investigating the best way to describe and express the relationships between organisms. On the other hand, it supplies the means for communication between biologists in all fields. For the most part, these two aims compliment each other, but sometimes they can clash. The first aim implies continual change, as our understanding of the relationships between organisms changes and (hopefully) improves. Wheeler (2007) commented in a recent editorial that "Doing taxonomy as an independent science advances simultaneously both the aims
of taxonomy and its users", a sentiment that I agree with fully (be warned, though, that the general tone of Wheeler's editorial is fairly incendiary). To fulfil the second aim, however, a certain amount of stability is usually desired, as researchers who are not working in taxonomy may have trouble keeping up with the changes (or, for that matter, appreciating their necessity).

All of the codes of nomenclature have a central commission to regulate taxonomy - zoology has the International Commission on Zoological Nomenclature, botany has the International Association for Plant Taxonomy. One of the main roles of these commissions is to allow suspension of the usual rules in cases where their strict application would cause more trouble for communication than otherwise. In the case of zoology, applications for rulings on such cases that are submitted to the ICZN are published in the journal Bulletin on Zoological Nomenclature, allowing researchers the opportunity to comment on submissions before the Commission decides on them. One submission that appeared in the December 2007 issue of the BZN involves a case that could affect a large number of researchers in many fields - the impending revision of the fly genus Drosophila.

Drosophila is a very large genus, containing about 1500 species. However, phylogenetic studies (e. g. Robe et al., 2005) have found that Drosophila as currently defined is significantly paraphyletic with regard to a number of other genera in the family Drosophilidae. There are two options to resolve this situation. One is to sink all the smaller genera arising from Drosophila into the larger genus. However, this is not regarded as a suitable solution - not only would it leave Drosophila with over 2000 species, but it would result in over a hundred secondary homonyms (two or more species ending up with the same name as a result of change in genus assignment) that would require correction. The other option, that seems much more likely to be used, is to divide Drosophila into a number of smaller genera. The name Drosophila would then be restricted to a smaller group of species closely related to the type species.

All this would be fairly routine, except that one of the species affected happens to be one of the most widely used model organisms in genetics - the "fruit fly" Drosophila melanogaster (the inverted commas are because Drosophila isn't really a fruit fly proper, but a vinegar fly). So familiar is this species that many people simply refer to it as Drosophila without invoking the species name. One might be forgiven for expecting D. melanogaster to be the type species of Drosophila, but it's not. That honour goes to Drosophila funebris (shown at the top of the post in a photo from here). And as it happens, the two species are not that closely related. If Drosophila is divided up, the Drosophila melanogaster everyone knows and loves becomes a far less familiar Sophophora melanogaster. How will geneticists respond to the loss of their favourite organism?

To avert an apocalypse in evolutionary biology, van der Linde et al. (2007) have made a submission to the ICZN to redefine the type species of Drosophila. They suggest that that honour be given to D. melanogaster rather than D. funebris, meaning that D. melanogaster would remain forever more Drosophila. But if this is accepted, what will become of D. funebris and its close friends and relatives? Will the ICZN exalt D. melanogaster to the position of type species? Or will the geneticists just have to learn to refer to Sophophora, and like it?

REFERENCES

Linde, K. van der, G. Bächli, M. J. Toda, W.-X. Zhang, Y.-G. Hu & G. S. Spicer. 2007. Case 3407: Drosophila Fallén, 1832 (Insecta, Diptera): proposed conservation of usage. Bulletin of Zoological Nomenclature 64 (4).

Robe, L. J., V. L. S. Valente, M. Budnik & E. L. S. Loreto. 2005. Molecular phylogeny of the subgenus Drosophila (Diptera, Drosophilidae) with an emphasis on Neotropical species and groups: a nuclear versus mitochondrial gene approach. Molecular Phylogenetics and Evolution 36: 623-640.

Wheeler, Q. D. 2007. Invertebrate systematics or spineless taxonomy? Zootaxa 1668: 11-18.

A Queenage of Strepsiptera

Those of you wondering about the significance of the title to this post might want to check out the comments for last week's post on Embioptera. I noted there that a collective noun for Strepsiptera would arguably be one of the most useless concepts in the English language. In making that comment, I was referring to the fact that Strepsiptera, to the best of my knowledge, pretty never occur in noticeable groups. In fact, Strepsiptera are one of the rarest of all insect orders - so rare as to be almost mythical*. As such, their existence is not widely known by non-entomologists, and the discovery of a strepsipteran specimen is usually heralded by an unsuspecting research assistant looking down a microscope at a dish of unsorted survey specimens suddenly exclaiming, "What the f*** is that?"

*If you want a more concrete example, an ecological survey being conducted by colleagues of mine has so far collected tens of thousands of specimens - including about three strepsipterans.

Strepsiptera are endoparasites of other insects. The name means 'twisted wing', and you may also find them being called stylops*. Both sexes are parasitic as larvae, and after pupating the winged males leave the host in search of females (the picture above, from Tree of Life, shows a male Pseudoxenos leaving its wasp host. Ick). Mature males never feed, and may only survive for a few hours. The females, except for one primitive family, never leave the host, but remain in a larva-like form.

*By the way, 'stylops' is both the singular and the plural.



In the extremely unlikely event of ever seeing a strepsipteran, you can rest assured that they cannot be easily mistaken for anything else. The picture above comes from here, and shows a generalised strepsipteran male. Strepsiptera have only one pair of functional wings, with the front pair reduced to balancing organs called halteres. The only other insect order to possess halteres are Diptera (flies), but in Diptera it is the hind pair that has been altered (more on that later). The antennae are branched and antler-like. The so-called 'raspberry eye' of Strepsiptera is actually unique in the insect world, with many disjoint ocelli. It can be seen better in the photo below of Caenocholax fenyesi (by Steve Taylor, from here).



The larvae are produced viviparously by the female, and emerge from the host in large numbers (so maybe there is a use for the collective noun, after all). The first instar larvae (known as triungulins) are surprisingly advanced, with well-developed eyes and legs in order to seek out a new host. Once they have found a host and burrowed in, however, all these mod-cons are jettisoned, leaving the larva legless and grub-like. The presence of such distinct larval stages is referred to as hypermetamorphosis. At least one strepsipteran family, the Myrmecolacidae, has particularly unusual host preferences - the males are parasites of ants, while the females favour grasshopppers and crickets (Kathrithamby et al., 2003). I have not been able to find whether the sex of the larva determines the host, or whether the host determines the sex.

Phylogenetically, the Strepsiptera are arguably the second most difficult insect order - probably, only the Zoraptera can claim to have caused more problems. Still, there are two main competitors for the position of nearest strepsipteran relative. For a long time, the Strepsiptera were associated with the beetles, to the extent that some authors even suggested reducing them to a subgroup of the Coleoptera. This was mainly predicated on similarities between the triungulin larvae of Strepsiptera and certain Coleoptera families, some of which shared the Strepsiptera's branched antennae and hypermetamorphosis. However, these features are also found in other unrelated insect groups, and the chance of convergence cannot be dismissed. Molecular analyses, on the other hand, suggested a relationship between Strepsiptera and Diptera, leading to the radical suggestion by Whiting & Wheeler (1994) that the strepsipteran halteres might actually be homologous to those of Diptera, and their difference in position might be due to a homoeotic reversal switching the identities of the wing pairs! At present, it is difficult to imagine how such a thing could have happened without fatally scrambling the rest of the insect's anatomy in that area, and even if they are sister groups, the Strepsiptera and Diptera may have still evolved their respective halteres independently.

Male Stylops pacificus mating with female parasitic on bee. Photo by Edward Ross, from Tree of Life.

And why should a collection of Strepsiptera be called a 'queenage'? It should be noted that parasitism by Strepsiptera (known as stylopisation), despite the inherent ickiness of having a grub-like parasite protruding from your abdomen, is rarely fatal, and males and larvae can emerge without harming the host. Indeed, stylopised hosts may live longer than they would normally. However, stylopisation can have other significant consequences. Gonad development is reduced, and stylopised hosts may often be sterile. Stylopisation may also have a dramatic effect on secondary sexual characteristics of the host - stylopised individuals may lose their expected secondary sexual features and develop features characteristic of the other sex (Salt, 1927). Hughes et al. (2004) discovered that stylopised individuals of one species of wasp did not work in the colony as normal, but abandoned the colony and formed loose aggregations elsewhere.

Parasite-induced castration is not uncommon in invertebrates, and it is believed that it is advantageous for the parasite to sterilise its host because then time and energy that the host would otherwise waste on finding and winning a mate and producing offspring can instead be focused on feeding the host and hence the parasite (think about the behavioural differences between a neutered and entire cat). Colony desertion by stylopised wasps is probably also induced by the parasite (stylopised individuals were not driven away from the colony by uninfected individuals) as the chance of successful male emergence and mating was greater in the aggregations than within the nest, where healthy wasps would destroy any male strepsipterans they spotted.

REFERENCES

Hughes, D. P., J. Kathirithamby, S. Turillazzi & L Beani. 2004. Social wasps desert the colony and aggregate outside if parasitized: parasite manipulation? Behavioral Ecology 15 (6): 1037-1043.

Kathirithamby, J., L. D. Ross & J. C. Johnston. 2003. Masquerading as self? Endoparasitic Strepsiptera (Insecta) enclose themselves in host-derived epidermal bag. Proceedings of the National Academy of Sciences of the USA 100 (13): 7655-7659.

Salt, G. 1927. The effects of stylopization on aculeate Hymenoptera. Journal of Experimental Zoology 48: 223-331.

Whiting, M. F., & W. C. Wheeler. 1994. Insect homeotic transformation. Nature 368: 696.

What is a Daddy-Longlegs?

"Daddy-longlegs" is one of the worst animal names there is. The name is widely used and generally recognised, but causes endless confusion because it is applied to no less than three very different animals. In a brief attempt at disambiguation, these are the animals involved:

First are harvestmen of the order Opiliones (picture from UMMZ). Harvestmen are often confused with spiders, but the body is not divided into a cephalothorax and abdomen, the opisthosoma (the posterior part of the body corresponding to the abdomen) is externally segmented, the chelicerae (mouthparts) are pincers rather than fangs, and harvestmen do not produce silk. The name "daddy-longlegs" as applied to harvestmen usually refers to the group known as "long-legged harvestmen" (Palpatores). There is some uncertainty about whether Palpatores are a monophyletic group, but that's a subject for another time.

Second are spiders of the family Pholcidae (picture is from Iziko Museums of Cape Town - the object the spider is holding is the egg-sac, which is carried by the female until the eggs hatch). Pholcids are true spiders, and so have a divided body, an unsegmented abdomen, fangs, and produce silk. Here in Australia and New Zealand, the 'daddy-longlegs' that are almost ubiquitously found in houses (particularly bathrooms) are pholcids, most often the introduced Pholcus phalangioides. Offhand, there is a common belief that daddy-longlegs (either pholcids or Opiliones) are "the most poisonous spiders in existence, but their fangs are too small to pierce human skin". I have come across this story many times, and have even been assured of it by people who really should know better. This story is absolute bunkum. The University of California, Riverside site has more info.

Finally, the third group accused of being 'daddy-longlegs' are crane flies of the family Tipulidae (picture from Wikipedia). Crane flies look a bit like giant mosquitoes, but they are not blood-suckers. They are a large family - the adults are nectarivores or do not feed, while the larvae, commonly called leatherjackets, feed on vegetation. Crane flies are easily distinguished from the other 'daddy-longlegs' - the wings are a bit of a give-away.

Insects Never Fail to Amaze

Recently I saw my first ever specimen of Archaeognatha. I was going to write on that, so I picked up the lab's faithful copy of The Insects of Australia (CSIRO, 1991) to look up information. Before I found the Archaeognatha chapter, however, I came across something else that just blew me away so much that I had to share it with you all. Meet the freaky little marine midge Pontomyia (the only image I could find online was a rather blurry one here. Sorry).

There are very few marine insects, and only a single genus, the waterstrider Halobates, has species that are actually found on the open ocean (van der Hage, 1996). Other marine insects are restricted to inshore habitats. Pontomyia appears to be an inhabitant of tide pools and lagoons in the West Pacific. It belongs to the large family Chironomidae, mosquito-like (but non-parasitic) midges with aquatic larvae

Individuals of Pontomyia spend most of their lives as benthic larvae. After they emerge as non-feeding adults, they only live for a couple of hours (Soong et al., 1999). In this brief time, they must find a mate and produce eggs.

Pontomyia adults emerge at dusk or after sunset. At least one species, Pontomyia oceana, only emerges around the new and full moons (in combination with the specific emergence time, this probably ensures that the females end up laying eggs at low tide). Pupae swim to the surface and emerge as adults. The females are vermiform and structurally degenerate, with seemingly little activity as far as I can tell.

The males are the freaky ones. They skim the water surface film on the tips of the stout second and stilt-like, trailing third pairs of legs. The first pair of legs is immensely long and curve out on either side of the body as a pair of 'outriggers', barely skimming the surface and maintaining the animal's balance. The paddle-like wings propel the midge by flicking the air just above the water surface (Norris, 1991).

Females do not complete emergence from the pupa unless males are nearby (Soong et al., 1999). Males generally emerge up to an hour before females, and have been observed stripping the pupal skin from females to help them emerge. Once a male has found a female, he picks her up with the second legs and the base of the third legs and carries her while mating. Males are apparently quick movers, and I was especially taken by this sentence in Soong et al. (1999): 'They did not appear to slow down after catching females, sometimes climbing the vertical substrate up to 15 cm above the water level while dragging a female along'. One can't help wondering what Germaine Greer would make of the verbs in there.

After mating, the male drops the female. She lays her eggs on bits of dead coral or the like sticking up about the water surface in long interconnected strings. And that, as they say, is that.

REFERENCES

Norris, K. R. 1991. General biology. In The Insects of Australia (CSIRO, eds.), 2n ed., vol. I pp. 68-108. Melbourne University Press.

Soong, K., G.-F. Chen & J.-R. Cao. 1999. Life history studies of the flightless marine midges Pontomyia spp. (Diptera: Chironomidae). Zoological Studies 38 (4): 466-473. (Pdf here)

van der Hage, J. C. H. 1996. Why are there no insects and so few higher plants, in the sea? New thoughts on an old problem. Functional Ecology 10: 546-547.