What I’ve been reading… week of March 4th, 2013

1) Inward, et al. (2007). Death of an order: a comprehensive molecular phylogenetic study confirms that termites are eusocial cockroachesBiol. Lett. vol. 3 no. 3 331-335


Termites are instantly recognizable mound-builders and house-eaters: their complex social lifestyles have made them incredibly successful throughout the tropics. Although known as ‘white ants’, they are not ants and their relationships with other insects remain unclear. Our molecular phylogenetic analyses, the most comprehensive yet attempted, show that termites are social cockroaches, no longer meriting being classified as a separate order (Isoptera) from the cockroaches (Blattodea). Instead, we propose that they should be treated as a family (Termitidae) of cockroaches. It is surprising to find that a group of wood-feeding cockroaches has evolved full sociality, as other ecologically dominant fully social insects (e.g. ants, social bees and social wasps) have evolved from solitary predatory wasps.

Termites are definitely eusocial cockroaches, and evolved sociality after the ability to digest wood (aided by mutualistic symbionts). Neat!

2) Gould and Lewontin (1979). The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proc. R. Soc. Lond. B 21 September 1979 vol. 205 no. 1161 581-598


An adaptationist programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an organism into unitary ‘traits’ and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the only brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analysed as integrated wholes, with Bauplane so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of non-adaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. We support Darwin’s own pluralistic approach to identifying the agents of evolutionary change.

I’m in the process of writing a qualifying exam, and one of the questions is about the idea of evolutionary constraints. Hence this classic paper about rejecting a purely adaptationist idea of evolution… Natural Selection is not the only force that shapes evolution. They suggest that when researchers are trying to answer any question of how a character evolved they need to seriously consider the impact of Drift, Constraint and Natural Selection. That said, constraint is a tricky concept… more on that later?

3) Ullrich-Lüter, et al. 2011. Unique system of photoreceptors in sea urchin tube feetPNAS 2011 108 (20) 8367-8372; doi:10.1073/pnas.1018495108


Different sea urchin species show a vast variety of responses to variations in light intensity; however, despite this behavioral evidence for photosensitivity, light sensing in these animals has remained an enigma. Genome information of the recently sequenced purple sea urchin (Strongylocentrotus purpuratus) allowed us to address this question from a previously unexplored molecular perspective by localizing expression of the rhabdomeric opsin Sp-opsin4 and Sp-pax6, two genes essential for photoreceptor function and development, respectively. Using a specifically designed antibody against Sp-Opsin4 and in situ hybridization for both genes, we detected expression in two distinct groups of photoreceptor cells (PRCs) located in the animal’s numerous tube feet. Specific reactivity of the Sp-Opsin4 antibody with sea star optic cushions, which regulate phototaxis, suggests a similar visual function in sea urchins. Ultrastructural characterization of the sea urchin PRCs revealed them to be of a microvillar receptor type. Our data suggest that echinoderms, in contrast to chordates, deploy a microvillar, r-opsin–expressing PRC type for vision, a feature that has been so far documented only in protostome animals. Surprisingly, sea urchin PRCs lack any associated screening pigment. Indeed, one of the tube foot PRC clusters may account for directional vision by being shaded through the opaque calcite skeleton. The PRC axons connect to the animal internal nervous system, suggesting an integrative function beyond local short circuits. Because juveniles display no phototaxis until skeleton completion, we suggest a model in which the entire sea urchin, deploying its skeleton as PRC screening device, functions as a huge compound eye.

Although light responses by various echinoderms has been known for a long time, this paper describes the cells that underlie some of those response in the purple urchin Strongylocentrotus purpatus. Based on their positioning, it seems as though these cells use the echinoderm test as screening pigment, letting them get light from only one part of space, and conferring a low-res spatial vision (see Yerramilli & Johnsen, 2009)From the genome we knew that sea urchins had 6 opsins, including both c-opsin and r-opsin and no eyes, so the obvious question is what are all those opsins doing? The behavioral experiments are buried in the back of the supplemental data, but the urchins were negatively phototactic, maximum reaction near 450nm (which is pretty blue).  Hopefully we’ll hear about the other opsins eventually. It’s going to be hard to prove the function of these photoreceptor cells, as echinoderms are probably the worst possible animals for electrophysiology.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s