Specification of left-right asymmetry in the embryonic gut of Drosophila.

Most animals exhibit stable left-right asymmetries in their body. Although significant progress has been made in elucidating the mechanisms that set up these asymmetries in vertebrates, nothing is known about them in Drosophila. This is usually attributed to the fact that no reversals of stable left-right asymmetries have been observed in Drosophila, although relevant surveys have been carried out. We have focused on the asymmetry of the proventriculus in the embryonic gut of Drosophila, an aspect of left-right asymmetry that is extremely stable in wild-type flies. We show that this asymmetry can be reversed by mutations in the dicephalic and wunen genes, which also cause reversals in the antero-posterior axis of the embryo relative to its mother. This is the first observation to suggest that left-right asymmetries in Drosophila can be reversed by genetic/developmental manipulations. It also suggests that maternal signals may initiate the specification of some left-right asymmetries in the embryo.

Evidence for a high frequency of simultaneous double-nucleotide substitutions.

Point mutations are generally assumed to involve changes of single nucleotides. Nevertheless, the nature and known mechanisms of mutation do not exclude the possibility that several adjacent nucleotides may change simultaneously in a single mutational event. Two independent approaches are used here to estimate the frequency of simultaneous double-nucleotide substitutions. The first examines switches between TCN and AGY (where N is any nucleotide and Y is a pyrimidine) codons encoding absolutely conserved serine residues in a number of proteins from diverse organisms. The second reveals double-nucleotide substitutions in primate noncoding sequences. These two complementary approaches provide similar high estimates for the rate of doublet substitutions, on the order of 0.1 per site per billion years.

Crustacean appendage evolution associated with changes in Hox gene expression.

Homeotic (Hox) genes specify the differential identity of segments along the body axis of insects. Changes in the segmental organization of arthropod bodies may therefore be driven by changes in the function of Hox genes, but so far this has been difficult to demonstrate. We show here that changes in the expression pattern of the Hox genes Ubx and AbdA in different crustaceans correlate well with the modification of their anterior thoracic limbs into feeding appendages (maxillipeds). Our observations provide direct evidence that major morphological changes in arthropod body plans are associated with changes in Hox gene regulation. They suggest that homeotic changes may play a role in the normal process of adaptive evolutionary change.

Evolutionary origin of insect wings from ancestral gills.

Two hypotheses have been proposed for the origin of insect wings. One holds that wings evolved by modification of limb branches that were already present in multibranched ancestral appendages and probably functioned as gills. The second proposes that wings arose as novel outgrowths of the body wall, not directly related to any pre-existing limbs. If wings derive from dorsal structures of multibranched appendages, we expect that some of their distinctive features will have been built on genetic functions that were already present in the structural progenitors of insect wings, and in homologous structures of other arthropod limbs. We have isolated crustacean homologues of two genes that have wing-specific functions in insects, pdm (nubbin) and apterous. Their expression patterns support the hypothesis that insect wings evolved from gill-like appendages that were already present in the aquatic ancestors of both crustaceans and insects.

DNA sequence evolution: the sounds of silence.

Silent sites (positions that can undergo synonymous substitutions) in protein-coding genes can illuminate two evolutionary processes. First, despite being silent, they may be subject to natural selection. Among eukaryotes this is exemplified by yeast, where synonymous codon usage patterns are shaped by selection for particular codons that are more efficiently and/or accurately translated by the most abundant tRNAs; codon usage across the genome, and the abundance of different tRNA species, are highly co-adapted. Second, in the absence of selection, silent sites reveal underlying mutational patterns. Codon usage varies enormously among human genes, and yet silent sites do not appear to be influenced by natural selection, suggesting that mutation patterns vary among regions of the genome. At first, the yeast and human genomes were thought to reflect a dichotomy between unicellular and multicellular organisms. However, it now appears that natural selection shapes codon usage in some multicellular species (e.g. Drosophila and Caenorhabditis), and that regional variations in mutation biases occur in yeast. Silent sites (in serine codons) also provide evidence for mutational events changing adjacent nucleotides simultaneously.

Hox genes and the diversification of insect and crustacean body plans.

Crustaceans and insects share a common origin of segmentation, but the specialization of trunk segments appears to have arisen independently in insects and various crustacean subgroups. Such macroevolutionary changes in body architecture may be investigated by comparative studies of conserved genetic markers. The Hox genes are well suited for this purpose, as they determine positional identity along the body axis in a wide range of animals. Here we examine the expression of four Hox genes in the branchiopod crustacean Artemia franciscana, and compare this with Hox expression patterns from insects. In Artemia the three 'trunk' genes Antp, Ubx and abdA are expressed in largely overlapping domains in the uniform thoracic region, whereas in insects they specify distinct segment types within the thorax and abdomen. Our comparisons suggest a multistep process for the diversification of these Hox gene functions, involving early differences in tissue specificity and the later acquisition of a role in defining segmental differences within the trunk. We propose that the branchiopod thorax may be homologous to the entire pregenital (thoracic and abdominal) region of the insect trunk.

Arthropod evolution: great brains, beautiful bodies.

While arthropod phylogeny remains controversial, comparative studies of the genetic control of segmentation and of the nervous system have begun to throw light on how mandibulate arthropods (myriapods, crustaceans and insects) reached their current level of morphological and behavioural complexity. Insects and crustaceans show remarkable similarities in the construction of their brains, suggesting that their common ancestor had typically arthropod behaviour, while developmental genetic studies are consistent with this ancestor having had distinct head, trunk and tail regions. This conclusion contrasts with the influential view, drawn from comparative embryology and functional anatomy, that insects and crustaceans evolved independently from a simple worm-like organism, perhaps resembling an annelid.

The evolving role of Hox genes in arthropods.

Comparisons between Hox genes in different arthropods suggest that the diversity of Antennapedia-class homeotic genes present in modern insects had already arisen before the divergence of insects and crustaceans, probably during the Cambrian. Hox gene duplications are therefore unlikely to have occurred concomitantly with trunk segment diversification in the lineage leading to insects. Available data suggest that domains of homeotic gene expression are also generally conserved among insects, but changes in Hox gene regulation may have played a significant role in segment diversification. Differences that have been documented alter specific aspects of Hox gene regulation within segments and correlate with alterations in segment morphology rather than overt homeotic transformations. The Drosophila Hox cluster contains several homeobox genes that are not homeotic genes--bicoid, fushi-tarazu and zen. the role of these genes during early development has been studied in some detail. It appears to be without parallel among the vertebrate Hox genes. No well conserved homologues of these genes have been found in other taxa, suggesting that they are evolving faster than the homeotic genes. Relatively divergent Antp-class genes isolated from other insects are probably homologues of fushi-tarazu, but these are almost unrecognisable outside of their homeodomains, and have accumulated approximately 10 times as many changes in their homeodomains as have homeotic genes in the same comparisons. They show conserved patterns of expression in the nervous system, but not during early development.

HOM/Hox genes of Artemia: implications for the origin of insect and crustacean body plans.

BACKGROUND: Insects and crustaceans are generally assumed to derive from a segmented common ancestor that had a distinct head but uniform, undifferentiated trunk segments. The subdivision of the body into functionally distinct regions (e.g. thorax and abdomen) is thought to have evolved independently in these two lineages. In insects, the differences between segments in the trunk are controlled by the Antennapedia-like genes of the homeotic gene clusters. Study of these genes in crustaceans should provide a basis for comparing body plans and assessing their evolutionary origin. RESULTS: Using a polymerase chain reaction (PCR) / inverse PCR strategy, we have isolated six genes of the HOM/Hox family from the crustacean Artemia franciscana. Five of these are clearly identifiable as specific homologues of the insect homeotic genes Dfd, Scr, Antp, Ubx and abdA. The sixth appears to have no close counterpart in insects. CONCLUSION: All the homeotic genes that specify middle body regions in insects originated before the divergence of the insect and crustacean lineages, probably not later than the Cambrian (about 500 million years ago). A commonly derived groundplan may underlie segment diversity in these two groups.