The study of balancing selection, as a selective force maintaining adaptive genetic variation in gene pools longer than expected by drift, is currently experiencing renewed interest due to the increased availability of new data, methods of analysis, and case studies. In this investigation, evidence of balancing selection operating on conserved enhancers of the olfactory receptor (OR) genes is presented for the Chinese sleeper (Bostrychus sinensis), a coastal marine fish that is emerging as a model species for evolutionary studies in the Northwest Pacific marginal seas. Coupled with tests for Gene Ontology enrichment and transcription factor binding, population genomic data allow for the identification of an OR cluster in the sleeper with a downstream flanking region containing three enhancers that are conserved with human and other fish species. Phylogenetic and population genetic analyses indicate that the enhancers are under balancing selection as evidenced by their translineage polymorphisms, excess common alleles, and increased within-group diversities. Age comparisons between the translineage polymorphisms and most recent common ancestors of neutral genealogies substantiate that the former are old, and thus, due to ancient balancing selection. The survival and reproduction of vertebrates depend on their sense of smell, and thereby, on their ORs. In addition to locus duplication and allelic variation of structural genes, this study highlights a third mechanism by which receptor diversity can be achieved for detecting and responding to the huge variety of environmental odorants (i.e., by balancing selection acting on OR gene expression through their enhancer variability).
The purpose of this study was to sequence and characterize the endothelin receptors (ETRs) from the gill of the killifish, Fundulus heteroclitus, where the ET signaling cascade is hypothesized to be involved in control of local blood flow and ion transport. In mammals there are two ETRs termed ETA and ETB. Interestingly, in non-mammalian vertebrates there are three ETRs: ETA, ETB1, and ETB2 (ETC in frogs). Using standard cloning and sequencing, we have sequenced the three ETRs from killifish gill cDNA. Our phylogenetic analysis supports previous findings that the three ETRs are produced by separate genes, and are not simply splice variants. To further characterize these receptors, tissue distribution and quantitative PCR mRNA analyses were performed. Given that animals from fishes to birds have three ETRs while mammals have only two, we hypothesize that mammals have lost the ETB2 gene. Supported by NSF Grants IBN-008942 and IOB-056273 to DHE and Sgima Xi GIAR to KAH.
Ca(2+)-activated Cl(-) channels (CaCCs) are critical to processes such as epithelial transport, membrane excitability, and signal transduction. Anoctamin, or TMEM16, is a family of 10 mammalian transmembrane proteins, 2 of which were recently shown to function as CaCCs. The functions of other family members have not been firmly established, and almost nothing is known about anoctamins in invertebrates. Therefore, we performed a phylogenetic analysis of anoctamins across the animal kingdom and examined the expression and function of anoctamins in the genetically tractable nematode Caenorhabditis elegans. Phylogenetic analyses support five anoctamin clades that are at least as old as the deuterostome/protosome ancestor. This includes a branch containing two Drosophila paralogs that group with mammalian ANO1 and ANO2, the two best characterized CaCCs. We identify two anoctamins in C. elegans (ANOH-1 and ANOH-2) that are also present in basal metazoans. The anoh-1 promoter is active in amphid sensory neurons that detect external chemical and nociceptive cues. Within amphid neurons, ANOH-1::GFP fusion protein is enriched within sensory cilia. RNA interference silencing of anoh-1 reduced avoidance of steep osmotic gradients without disrupting amphid cilia development, chemotaxis, or withdrawal from noxious stimuli, suggesting that ANOH-1 functions in a sensory mode-specific manner. The anoh-2 promoter is active in mechanoreceptive neurons and the spermatheca, but loss of anoh-2 had no effect on motility or brood size. Our study indicates that at least five anoctamin duplicates are evolutionarily ancient and suggests that sensory signaling may be a basal function of the anoctamin protein family.
Understanding rates and patterns of ribosomal RNA (rRNA) nucleotide change is important for understanding RNA structure/function relationships ( Brimacombe et al. 1986; Stem et al. 1988), for predicting higher-order rRNA tertiary and RNAprotein interactions (Zuker and Stiegler 198 1; Freier et al. 1986 ), for studies of rRNA molecular evolution (Gerbi 1985; Gutell et al. 1985; Rousset et al. 1991), and for accurate phylogenetic analysis based on rRNA primary sequences (Wheeler and Honeycutt 1988; Smith 1989). rRNA secondary structure is highly conserved across widely divergent taxa, despite considerable variation at the primary-sequence level (Gerbi 1985; Gutell et al. 1985; Engberg et al. 1990). Within a particular rRNA gene, variation in the rate of primary-sequence evolution exists, with single-stranded loops evolving more rapidly than do presumed double-stranded stems (Tanhauser 1985). This result is expected if the complementary base for a particular stem position exerts some constraint on the allowable evolution in its opposite, paired member. Such constraint has been demonstrated for the 5s rRNA gene, where evolution in the stem regions is highly directed (Wheeler and Honeycutt 1988). Constraint on the evolutionary freedom of rRNA stem positions may be assessed by investigating the degree to which changes in one nucleotide of a stem base pair are compensated in the other member of the base pair (Wheeler and Honeycutt 1988). In addition, characterizing persistent mispairings may provide some insights into selective pressures on rRNA molecules. Here we present such an investigation for the 12s rRNA genes of the mitochondrial DNA (mtDNA) from an assortment of artiodactyls. This order of eutherian mammals has a diversity of extant and fossil species, thereby presenting a series of relatively well-dated cladogenetic events covering a large time range and several hierarchical taxonomic levels (Miyamoto et al., accepted). The tree topology and times of divergence used to study the evolution of artiodactyl 12s rRNAs are shown in figure 1. The topology for bovines (Miyamoto et al. 1989), cervids (Miyamoto et al. 1990)) and higher-level pecoran relationships (Simpson 1945; Janis and Scott 1987; Gentry and Hooker 1988) is based only on unambiguously well-diagnosed lineages. Dates of hypothetical common ancestors are those justified previously by Miyamoto et al. (accepted). The sequences themselves are from previous studies (Anderson et al. 1982; Tanhauser 1985; Miyamoto et al. 1990; Kraus and Miyamoto 199 1) . Fifty-one stems were identified by using the secondary-structure model of Gutell et al. ( 1985), except that eight pairs of stems, separated by a single base pair in that model, were each joined together to form eight composite stems, for a total of 43 stems. Twenty-nine of these stems contained 79 pairs of nucleotides that were variable in at least one taxon. We optimized these 79 nucleotide pairs on the tree of figure 1 by using the computer program PAUP [ Phylogenetic Analysis Using Parsimony, ver-
Bazin et al. (Reports, 28 April, 2006, p. 570) found no relationship between mitochondrial DNA (mtDNA) diversity and population size when comparing across large groups of animals. We show empirically that species with smaller populations, as represented by eutherian mammals, exhibit a positive correlation between mtDNA and allozyme variation, suggesting that mtDNA diversity may correlate with population size in these animals.
'%3e%3cg id='b'%3e%3cpath id='a' stroke='%23000' stroke-width='2' d='M-6-25H6m-12 3H6m-12 3H6'/%3e%3cuse xlink:href='%23a' y='44'/%3e%3c/g%3e%3cpath stroke='%23fff' d='M0 17v10'/%3e%3ccircle r='12' fill='%23cd2e3a'/%3e%3cpath fill='%230047a0' d='M0-12A6 6 0 0 0 0 0a6 6 0 0 1 0 12 12 12 0 0 1 0-24z'/%3e%3c/g%3e%3cg transform='rotate(-123.69)'%3e%3cuse xlink:href='%23b'/%3e%3cpath stroke='%23fff' d='M0-23.5v3M0 17v3.5m0 3v3'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)