The three worker castes of Pheidole tepicana: minors, majors, and supermajors

Ant enthusiasts know Pheidole as a common genus where each nest has two distinct worker types: small minors and big majors. But a few odd species add one more: enormous supermajors. You can see all three in the photo above of the Arizona species Pheidole tepicana.

This afternoon, developmental biologists at McGill University and University of Arizona published a clever study in Science suggesting that supermajors, although they emerged independently in the course of evolution, make use of a similar underlying hormonal process. What’s more- and this is what’s really exciting- they found they could induce supermajor-like workers in typical species by adding a juvenile-hormone analog to developing larvae.

modified from Rajakumar et al (2012) Fig. 4.

What does this mean? Possibly, all Pheidole species have retained an ancestral potential to create supermajors. A few minor tweaks of hormonal regulation and the caste starts to emerge. These induced soldiers don’t to my eye look exactly like the real-world supermajors (not enough head, really), so I suspect supermajors in nature are developmentally fine-tuned more than suggested here. Still, that such a simple developmental change can lead to large differences in colony allometry is intriguing, a clue to how the 1000+ species in the genus managed to produce such a shocking richness in morphology in very short time.

Pheidole obtusospinosa - supermajor worker

Pheidole is a complicated ant, though, and I’m not sure we’ve defined the terminology of caste clearly enough that Rajakumar et al‘s interpretation can be called uncontroversial. What, really, is a “major”? In light of a conserved supermajor pathway, and the tremendous variation of form among species, it may be that “majors” in some large-headed species are actually supermajors, with the intermediary major caste lost.

Rajakumar et al show that artificial supermajors- defined partly by thoracic development- can be produced in two different dimorphic lineages. Solid evidence, but given the newly apparent fluidity of caste I’m less sure than before that we know what caste even means.  Pheidole still has hundreds of untested species, and supermajors in life differ from majors more in head than thoracic allometry. Consider variation of heads among the typical, dimorphic species:

Pheidole major heads, borrowed from www.antweb.org.

Still, Rajakumar et al is a significant paper. Like all good science, by showing us a little we see how much more we have to learn.


source: Rajendhran Rajakumar, Diego San Mauro, Michiel B. Dijkstra, Ming H. Huang, Diana E. Wheeler, Francois Hiou-Tim, Abderrahman Khila, Michael Cournoyea, and Ehab Abouheif. Science 6 January 2012: 79-82. [DOI:10.1126/science.1211451]