Organelle evolution: what's in a name?

Plastids are organelles derived from cyanobacterial endosymbionts and the evolutionary process that gave rise to them is well understood. Or is it? The complete genome sequence of a recently evolved photosynthetic body in Paulinella chromatophora is cause for reflection on the distinction between 'endosymbiont' and 'organelle', and how the boundaries between these terms can blur.     

Cellular evolution: what's in a mitochondrion?

Mitochondria and their relatives constitute a wide range of organelles, only some of which function in aerobic respiration. Mitochondrial remnants from different anaerobic lineages show a striking degree of functional convergence.     

MUC genes: a superfamily of genes? Towards a functional classification of human apomucins

The MUC genes encode epithelial mucins. Eight different human genes have been well characterized, and two others identified more recently. Among them, a family of four genes, expressed in the respiratory and digestive tracts, is clustered to chromosome 11p15.5; and these genes encode gel-forming mucins which are structurally related to the superfamily of cystine-knot growth factors. A second group is composed of three independent genes encoding various isoforms of mucins including membrane-bound mucins associated to carcinomas. In this second group, MUC3 and MUC4 encode large apomucins containing EGF-like domains.     

Expression of somite segmentation genes in amphioxus: a clock without a wavefront?

In the basal chordate amphioxus (Branchiostoma), somites extend the full length of the body. The anteriormost somites segment during the gastrula and neurula stages from dorsolateral grooves of the archenteron. The remaining ones pinch off, one at a time, from the tail bud. These posterior somites appear to be homologous to those of vertebrates, even though the latter pinch off from the anterior end of bands of presomitic mesoderm rather than directly from the tail bud. To gain insights into the evolution of mesodermal segmentation in chordates, we determined the expression of ten genes in nascent amphioxus somites. Five (Uncx4.1, NeuroD/atonal-related, IrxA, Pcdhdelta2-17/18, and Hey1) are expressed in stripes in the dorsolateral mesoderm at the gastrula stage and in the tail bud while three (Paraxis, Lcx, and Axin) are expressed in the posterior mesendoderm at the gastrula and neurula stages and in the tail bud at later stages. Expression of two genes (Pbx and OligA) suggests roles in the anterior somites that may be unrelated to initial segmentation. Together with previous data, our results indicate that, with the exception that Engrailed is only segmentally expressed in the anterior somites, the genetic mechanisms controlling formation of both the anterior and posterior somites are probably largely identical. Thus, the fundamental pathways for mesodermal segmentation involving Notch-Delta, Wnt/beta-catenin, and Fgf signaling were already in place in the common ancestor of amphioxus and vertebrates although budding of somites from bands of presomitic mesoderm exhibiting waves of expression of Notch, Wnt, and Fgf target genes was likely a vertebrate novelty. Given the conservation of segmentation gene expression between amphioxus and vertebrate somites, we propose that the clock mechanism may have been established in the basal chordate, while the wavefront evolved later in the vertebrate lineage.     

The evolution of ADHD: a disorder of communication?

Attention deficit hyperactivity disorder (ADHD) is the most commonly diagnosed psychiatric condition. Many believe that the central disability is impaired inhibition, which leads to reduced abilities in social skills, self-control, organization and time management. The behaviors identified by clinicians as problematic--inattention, hyperactivity and impulsivity--have been incorporated into several evolutionary models as selectively adaptive cognitive skills for surviving the challenges of a variable Pleistocene environment. We propose that the "disabilities" exhibited by individuals with ADHD are maladaptive, and we concur with Barkley that there is a central impairment in the behavioral inhibition system. The underlying neural anatomy and physiology support the possibility that neurotransmitter pathology may have an impact on other interlinked systems (including language), and may also account for the frequent comorbidity of aggression, anxiety, depression, and learning disabilities (many of which are language-related). Language skills compete with other cognitive activities for the attentional system, and thus the evolution of language could not in fact be independent of the evolution of attention. If language represents the ultimate expression of the attentional system, and some individuals with ADHD are seriously impaired in the coordination of interlinked neural systems (including language), then ADHD fits Jerome Wakefield's definition of "harmful dysfunction," and communication impairments should be investigated more thoroughly by clinicians.     

Gravity perception in plants: a multiplicity of systems derived by evolution?

The origin and subsequent evolution of life on Earth have taken place within an environment where a 1g gravitational field is omnipresent. Living organisms, at whatever stage in their evolution, have accommodated this variable in both their structure and their function. Systems have also evolved whereby gravitational accelerations are perceived by gravisensors and these, in turn, have led to responses that give particular spatial orientations to living processes. It is proposed that, the higher the evolutionary status of an organism, the more likely it is that it will possess multiple systems for gravisensing because evolution discards little that assists fitness and hence supplements with new gravisensing systems those which already existed within evolutionarily older, less complex organisms. Moreover, in comparison with a single gravisensing system, a multiplicity of systems permits gravity to participate in a wider range of developmental programmes, such as taxes, morphisms and tropisms, through the action of different sensory mechanisms coupled to distinct signalling and response pathways. Whatever the precise mechanism of graviperception in any given set of conditions, all may transduce the g-force by means of a membrane system. Transduction may involve the endoplasmic reticulum and thence the plasma membrane.     

Heritability of cellular radiosensitivity: a marker of low-penetrance predisposition genes in breast cancer?

Many inherited cancer-prone conditions show an elevated sensitivity to the induction of chromosome damage in cells exposed to ionizing radiation, indicative of defects in the processing of DNA damage. We earlier found that 40% of patients with breast cancer and 5%-10% of controls showed evidence of enhanced chromosomal radiosensitivity and that this sensitivity was not age related. We suggested that this could be a marker of cancer-predisposing genes of low penetrance. To further test this hypothesis, we have studied the heritability of radiosensitivity in families of patients with breast cancer. Of 37 first-degree relatives of 16 sensitive patients, 23 (62%) were themselves sensitive, compared with 1 (7%) of 15 first-degree relatives of four patients with normal responses. The distribution of radiosensitivities among the family members showed a trimodal distribution, suggesting the presence of a limited number of major genes determining radiosensitivity. Segregation analysis of 95 family members showed clear evidence of heritability of radiosensitivity, with a single major gene accounting for 82% of the variance between family members. The two alleles combine in an additive (codominant) manner, giving complete heterozygote expression. A better fit was obtained to a model that includes a second, rarer gene with a similar, additive effect on radiosensitivity, but the data are clearly consistent with a range of models. Novel genes involved in predisposition to breast cancer can now be sought through linkage studies using this quantitative trait.     

Sexual cannibalism in scorpions: fact or fiction?

Females of several scorpion species are currently deemed to be sexual cannibals, but this reputation is, however, largely built on anecdotal evidence from the older literature. Theoretical models predict that given the low rates of female-male encounter, males should allow themselves to be cannibalized after sperm transfer. The present paper examines the information available for a number of species to determine (a) whether sexual cannibalism is as widespread as previously suggested, and (b) whether it is likely to involve male self-sacrifice. Information on three scorpions currently regarded as sexual cannibals is presented: the bothriurids Bothriurus bonariensis (C.L. Koch) and Urophonius jheringii Pocock, and the buthid Leiurus quinquestriatus (Hemprich & Ehrenberg). No post-mating cannibalism was ever observed in these species, nor in another 19 representatives of four families. In L. quinquestriatus only two cases of cannibalism of the male by the female occurred, both in the beginning of courtship and after long periods of food deprivation. We suggest that if sexual cannibalism occurs in scorpions, it is most likely to be of an 'economic' type, where females perceive males as prey rather than mates, and not a 'self-sacrificial', post-mating cannibalism enhancing the fitness of the offspring of the male.     

Does genetic conflict drive rapid molecular evolution of nuclear transport genes in Drosophila?

The Segregation Distorter (SD) system of Drosophila melanogaster is one the best-characterized meiotic drive complexes known. SD gains an unfair transmission advantage through heterozygous SD/SD(+) males by incapacitating SD(+)-bearing spermatids so that virtually all progeny inherit SD. Segregation distorter (Sd), the primary distorting locus in the SD complex, is a truncated duplication of the RanGAP gene, a major regulator of the small GTPase Ran, which has several functions including the maintenance of the nucleocytoplasmic RanGTP concentration gradient that mediates nuclear transport. The truncated Sd-RanGAP protein is enzymatically active but mislocalizes to the nucleus where it somehow causes distortion. Here I present data consistent with the idea that wild-type RanGAP, and possibly other loci able to influence the RanGTP gradient, has been caught up in an ancient genetic conflict that predates the SD complex. The legacy of this conflict could include the unexpectedly rapid evolution of nuclear transport-related proteins, the accumulation of chromosomal inversions, the recruitment of gene duplications, and the turnover of repetitive sequences in the centric heterochromatin.     

How many genes in a genome?

Despite the current good level of annotation, the Drosophila genome still holds surprises. A recent study has added perhaps 2,000 genes to the predicted total, and raises a number of questions about how genome annotation data should be stored and presented.     

Reporter genes: too much of a good thing?

Advances in viral vector design and identification of new reporter genes have allowed the development of novel delivery systems. In the presence of reporter genes, cellular transduction frequency, expression of the gene of interest and phenotypic effects in cells expressing the gene under study can now be easily monitored both in vitro and in vivo. Moreover, the presence of unique cell markers allows for the enrichment of transduced cells for research studies or patient infusion. The ideal reporter gene product should be biologically inert and not influence the cell population under investigation. Recent reports suggest that reporter gene products may not be biologically benign.     

A high density of X-linked genes for general cognitive ability: a run-away process shaping human evolution?

The incidence of mental disability is 30% higher in males than in females. We have examined entries in the OMIM database that are associated with mental disability and for several other common defects. Our findings indicate that compared with the autosomes, the X chromosome contains a significantly higher number of genes that, when mutated, cause mental impairment. We propose that these genes are involved in the development of cognitive abilities and thus exert a large X-chromosome effect on general intelligence in humans. We discuss these conclusions with regard to the conservation of the vertebrate X-chromosomal linkage group and to human evolution.     

Symbiont genes in host genomes: fragments with a future?

While lateral transfer is the rule in the evolutionary history of bacterial and archaeal genes, events of transfer from prokaryotes to eukaryotes are rare. Germline-transmitted animal symbionts, such as Wolbachia pipientis, are well placed to participate in such transfers. In a recent issue of Science, Dunning Hotopp et al. identified instances of transfer of Wolbachia DNA to host genomes. It is unknown whether these transfers represent innovation in animal evolution.     

Commentary: When does understanding phenotypic evolution require identification of the underlying genes?

Adaptive evolution is fundamentally a genetic process. Over the past three decades, characterizing the genes underlying adaptive phenotypic change has revealed many important aspects of evolutionary change. At the same time, natural selection is often fundamentally an ecological process that can often be studied without identifying the genes underlying the variation on which it acts. This duality has given rise to disagreement about whether, and under what circumstances, it is necessary to identify specific genes associated with phenotypic change. This issue is of practical concern, especially for researchers who study nonmodel organisms, because of the often enormous cost and labor required to “go for the genes.” We here consider a number of situations and questions commonly addressed by researchers. Our conclusion is that although gene identification can be crucial for answering some questions, there are others for which definitive answers can be obtained without finding underlying genes. It should thus not be assumed that considerations of “empirical completeness” dictate that gene identification is always desirable.     

Stem-loop potential in MHC genes: a new way of evaluating positive Darwinian selection?

The domains of polymorphic major histocompatibility complex (MHC) proteins which interact with peptides and T-cell receptors are considered to have been under positive evolutionary selection pressure. Evidence for this is a high ratio of non-synonymous to synonymous mutations in the corresponding genomic domains. By this criterion snake venom phospholipaseA ₂ genes have also been under positive selection pressure. Recent studies of the latter genes indicate that positive selection has overridden an evolutionary pressure on base order which normally promotes the potential to extrude single-strand stem-loops from supercoiled duplex DNA (fold pressure). This has resulted in base order-dependent stem-loop potential being shifted to introns, which are highly conserved between species. It is now shown that, like snake venom phospholipaseA ₂ genes, the domains of polymorphic MHC genes which appear to have responded to positive selection pressure have decreased base order-dependent stem-loop potential. The evolutionary pressure to generate stem-loop potential (believed to be important for recombination) has been overridden less in exons under positive Darwinian selection. Thus, base order-dependent stem-loop potential shows promise as an independent indicator of positive selection.