Differential recruitment of limb patterning genes during development and diversification of beetle horns

Moczek AP, Rose DJ (2009) PNAS 106: 8992-8997.

The origins of novel complex phenotypes represent one of the most fundamental, yet largely unresolved, issues in evolutionary biology. Here we explore the developmental genetic regulation of beetle horns, a class of traits that lacks obvious homology to traits in other insects. Futhermore, beetle horns are remarkably diverse in their expression, including sexual dimorphism, male dimorphism, and interspecific differences in location of horn expressions. At the same time, beetle horns share aspects of their development with that of more traditional appendages. We used RNA interference-mediated gene function analysis of 3 cardinal insect appendage patterning genes, dachshund, homothorax and Distal-less, to investigate their role in development and diversification of beetle horns within and between species. Transcript depletion of all 3 patterning genes generated phenotypic effects very similar to those documented in previous studies that focused on general insect development. In addition, we found that Distal-less and homothorax, but not dachshund, regulate horn expression in aspecies-, sex-, body region-, and body size-dependent manner. Our results demonstrate differential co-option of appendage patterning genes during the evolution and radiation of beetle horns. Furthermore, our results illustrate that regulatory genes whose functions are otherwise highly conserved nevertheless retain the capacity to acquire additional functions, and that little phylogenetic distance appears necessary for the evolution of sex- and species-specific differences in these functions.

Biology of Subterranean Termites: Insights from Molecular Studies of Reticulitermes and Coptotermes

Vargo EL, Husseneder C (2009) Ann Rev Entomol 54: 379-403.

Molecular genetic techniques have made contributions to studies on subterranean termites at all levels of biological organization. Most of this work has focused on Reticulitermes and Coptotermes, two ecologically and economically important genera. DNA sequence data have significantly improved our understanding of the systematics and taxonomy of these genera. Techniques of molecular biology have provided important new insights into the process of caste differentiation. Population genetic markers, primarily microsatellites, have furthered our understanding of the life history, population biology, community ecology, and invasion biology of subterranean termites. Recent results on the behavioral ecology of subterranean termites reveal a picture different from long-held views, especially those concerning colony breeding structures and foraging ranges. As additional molecular tools and genomic resources become available, and as more subterranean termite researchers incorporate molecular techniques into their approaches, we can expect accelerating advances in all aspects of the biology of this group.

An amino acid polymorphism in the couch potato gene forms the basis for climatic adaptation in Drosophila melanogaster

Schmidt PS, Zhu C-T, Das J, Batavia M, Yang L, Eanes WF (2008) An amino acid polymorphism in the couch potato gene forms the basis for climatic adaptation in Drosophila melanogaster. PNAS 105: 16207-16211.

Diapause is the classic adaptation to seasonality in arthropods, and its expression can result in extreme lifespan extension as well as enhanced resistance to environmental challenges. Little is known about the underlying evolutionary genetic architecture of diapause in any organism. Drosophila melanogaster exhibits a reproductive diapause that is variable within and among populations; the incidence of diapause increases with more temperate climates and has significant pleiotropic effects on a number of life history traits. Using quantitative trait mapping, we identified the RNA-binding protein encoding gene couch potato (cpo) as a major genetic locus determining diapause phenotype in D. melanogaster and independently confirmed this ability to impact diapause expression through genetic complementation mapping. By sequencing this gene in samples from natural populations we demonstrated through linkage association that variation for the diapause phenotype is caused by a single Lys/Ile substitution in one of the six cpo transcripts. Complementation analyses confirmed that the identified amino acid variants are functionally distinct with respect to diapause expression, and the polymorphism also shows geographic variation that closely mirrors the known latitudinal cline in diapause incidence. Our results suggest that a naturally occurring amino acid polymorphism results in the variable expression of a diapause syndrome that is associated with the seasonal persistence of this model organism in temperate habitats.

The Origins of Genome Architecture

The Origins of Genome Architecture

Michael Lynch

Sinauer Associates, Inc.

With official genomic blueprints now available for hundreds of species, and thousands more expected in the near future, the field of biology has been forever transformed. Such readily accessible data have encouraged the proliferation of adaptive arguments for the evolution of gene and genomic futures, often with little or no attention being given to simpler and more powerful alternative explanations. By integrating the central observations from molecular biology and population genetics relevant to comparative genomics, Lynch shows why the details matter.

Presented in a nontechnical fashion, at both the population-genetic and molecular-genetic levels, this book offers a unifying explanatory framework for how the peculiar architectural diversity of eukaryotic genome and genes came to arise. Under Lynch's hypothesis, the genome-wide repatterning of eukaryotic gene structure, which resulted primarily from nonadaptive processes, provided an entirely novel resource from which natural selection could secondarily build new forms of organismal complexity.

Phenotypic plasticity in Drosophila pigmentation caused by temperature sensitivity of a chromatin regulator network.

Gibert J-M, Peronnet F, Schlötterer C (2007)

Phenotypic plasticity in Drosophila pigmentation caused by temperature sensitivity of a chromatin regulator network.

PLoS Genetics 3: e30.

Phenotypic plasticity is the ability of a genotype to produce contrasting phenotypes in different environments. Although many examples have been described, the responsible mechanisms are poorly understood. In particular, it is not clear how phenotypic plasticity is related to buffering, the maintenance of a constant phenotype against genetic or environmental variation. We investigate here the genetic basis of a particularly well described plastic phenotype: the abdominal pigmentation in female Drosophila melanogaster. Cold temperature induces a dark pigmentation, in particular in posterior segments, while higher temperature has the opposite effect. We show that the homeotic gene Abdominal-B (Abd-B) has a major role in the plasticity of pigmentation in the abdomen. Abd-B plays opposite roles on melanin production through the regulation of several pigmentation enzymes. This makes the control of pigmentation very unstable in the posterior abdomen, and we show that the relative spatio-temporal expression of limiting pigmentation enzymes in this region of the body is thermosensitive. Temperature acts on melanin production by modulating a chromatin regulator network, interacting genetically with the transcription factor bric-a-brac (bab), a target of Abd-B and Hsp83, encoding Hsp90. Genetic disruption of this chromatin regulator network increases the effect of temperature and the instability of the pigmentation pattern in the posterior abdomen. Colocalizations on polytene chromosomes suggest that BAB and these chromatin regulators cooperate in the regulation of many targets, including several pigmentation enzymes. We show that they are also involved in sex comb development in males and that genetic destabilization of this network is also strongly modulated by temperature for this phenotype. Thus, we propose that phenotypic plasticity of pigmentation is a side effect reflecting a global impact of temperature on epigenetic mechanisms. Furthermore, the thermosensitivity of this network may be related to the high evolvability of several secondary sexual characters in the genus Drosophila.

Genetic caste determination in termites: out of the shade but not from Mars

Crozier RH, Schlüns H (2008) Genetic caste determination in termites: out of the shade but not from Mars. BioEssays 30: 299-302.

Several ant species are known with genetic effects on caste determination but, for termites, the role of environment has been assumed to be omnipotent. Now Hayashi et al. report that commitment to the nymph and worker pathways in Reticulitermes speratus follows a simple model involving two alleles at a sex-linked locus. The spread of this system of genetic caste determination seems best explained by selection at the colony level. This remarkable system may be widely applicable throughout termites, although it cannot be universal, and may provide a window into causal aspects of the molecular biology of caste determination.

Juvenile hormone titers and caste differentiation in the damp-wood termite

Cornette R, Gotoh H, Koshikawa S, Miura T (2008) Juvenile hormone titers and caste differentiation in the damp-wood termite Hodotermopsis sjostedti (Isoptera, Termopsidae). J Insect Physiol: in press.
[doi:10.1016/j.jinsphys.2008.04.017]

Termites are social insects, presenting morphologically distinct castes, performing specific tasks in the colony. The developmental processes underlying caste differentiation are mainly controlled by juvenile hormone (JH). Although many fragmentary data support this fact, there was no comparative work on JH titers during the caste differentiation processes. In this study, JH titer variation was investigated using a liquid chromatography-mass spectrometry (LC-MS) quantification method in all castes of the Japanese damp-wood termite Hodotermopsis sjostedti, especially focusing on the soldier caste differentiation pathway, which was induced by treatment with a JH analog. Hemolymph JH titers fluctuated between 20 and 720 pg/μl. A peak of JH was observed during molting events for the pseudergate stationary molt and presoldier differentiation, but this peak was absent prior to the imaginal molt. Soldier caste differentiation was generally associated with high JH titers and nymph to alate differentiation with low JH titers. However, JH titer rose in females during alate maturation, probably in relation to vitellogenesis. In comparison, JH titer was surprisingly low in neotenics. On the basis of these results in both natural and artificial conditions, the current model for JH action on termite caste differentiation is discussed and re-appraised.