Gene Duplication and the Resolution of Adaptive Conflict

Gene duplication is known to play an important role in the evolution of novel protein functions. However, there is still much debate about the evolutionary mechanisms that are responsible for the initial retention and subsequent divergence of newly created gene duplicates (Lynch et al., 2001; Zhang, 2003; Lynch and Katju, 2004; Taylor and Raes, 2004). According to the neofunctionalization (NEOF) model of Ohno (1970), the functional redundancy of duplicated genes entails a relaxation of purifying selection that results in the accumulation of degenerative mutations in one gene copy, while the other duplicate copy continues to perform the essential tasks of the ancestral, single-copy gene. In the vast majority of cases, the redundant gene duplicate will eventually be rendered functionless by inactivating mutations. In a very small minority of cases, the redundant gene may escape this fate by fixing one or more mutations that fortuitously adapt the encoded protein to a new or modified function

Gene Duplication and the Resolution of Adaptive Conflict

Gene duplication is known to play an important role in the evolution of novel protein functions. However, there is still much debate about the evolutionary mechanisms that are responsible for the initial retention and subsequent divergence of newly created gene duplicates (Lynch et al., 2001; Zhang, 2003; Lynch and Katju, 2004; Taylor and Raes, 2004). According to the neofunctionalization (NEOF) model of Ohno (1970), the functional redundancy of duplicated genes entails a relaxation of purifying selection that results in the accumulation of degenerative mutations in one gene copy, while the other duplicate copy continues to perform the essential tasks of the ancestral, single-copy gene. In the vast majority of cases, the redundant gene duplicate will eventually be rendered functionless by inactivating mutations. In a very small minority of cases, the redundant gene may escape this fate by fixing one or more mutations that fortuitously adapt the encoded protein to a new or modified function

Using genome scans of DNA polymorphism to infer adaptive population divergence

Elucidating the genetic basis of adaptive population divergence is a goal of central importance in evolutionary biology. In principle, it should be possible to identify chromosomal regions involved in adaptive divergence by screening genome-wide patterns of DNA polymorphism to detect the locus-specific signature of positive directional selection. In the case of spatially separated populations that inhabit different environments or sympatric populations that exploit different ecological niches, it is possible to identify loci that underlie divergently selected traits by comparing relative levels of differentiation among large numbers of unlinked markers. In this review I first address the question of whether diversifying selection on polygenic traits can be expected to produce predictable patterns of allelic variation at the underlying quantitative trait loci (QTL), and whether the locus-specific effects of selection can be reliably detected against the genome-wide backdrop of stochastic variability. I then review different approaches that have been developed to identify loci involved in adaptive population divergence and I discuss the relative merits of model-based approaches that rely on assumptions about population structure vs. model-free approaches that are based on empirical distributions of summary statistics. Finally, I consider the evolutionary and functional insights that might be gained by conducting genome scans for loci involved in adaptive population divergence

Variation at tri- and tetranucleotide repeat microsatellite loci in the fruit bat genus \u3ci\u3eCynopterus\u3c/i\u3e (Chiroptera: Pteropodidae)

There is considerable uncertainty surrounding the taxonomic relationship between Cynopterus sphinx and C. brachyotis, and the status of the many named forms within C. sphinx (Storz & Kunz 1999). Polymorphic microsatellite markers for cynopterine fruit bats would greatly aid efforts to elucidate species boundaries and genetic correlates of morphological variation within species. To assess levels of variation in C. sphinx and C. brachyotis, microsatellite genotypes were obtained for a total of 731 bats (620 C. sphinx and 111 C. brachyotis)

Bohr effect and temperature sensitivity of hemoglobins from highland and lowland deer mice

An important means of physiological adaptation to environmental hypoxia is an increased oxygen (O2) affinity of the hemoglobin (Hb) that can help secure high O2 saturation of arterial blood. However, the trade-off associated with a high Hb-O2 affinity is that it can compromise O2 unloading in the systemic capillaries. High-altitude deer mice (Peromyscus maniculatus) have evolved an increased Hb-O2 affinity relative to lowland conspecifics, but it is not known whether they have also evolved compensatory mechanisms to facilitate O2 unloading to respiring tissues. Here we investigate the effects of pH (Bohr effect) and temperature on the O2-affinity of high- and low-altitude deer mouse Hb variants, as these properties can potentially facilitate O2 unloading to metabolizing tissues. Our experiments revealed that Bohr factors for the high- and low-altitude Hb variants are very similar in spite of the differences in O2-affinity. The Bohr factors of deer mouse Hbs are also comparable to those of other mammalian Hbs. In contrast, the high- and low-altitude variants of deer mouse Hb exhibited similarly low temperature sensitivities that were independent of red blood cell anionic cofactors, suggesting an appreciable endothermic allosteric transition upon oxygenation. In conclusion, high-altitude deer mice have evolved an adaptive increase in Hb-O2 affinity, but this is not associated with compensatory changes in sensitivity to changes in pH or temperature. Instead, it appears that the elevated Hb-O2 affinity in high-altitude deer mice is compensated by an associated increase in the tissue diffusion capacity of O2 (via increased muscle capillarization), which promotes O2 unloading

Life Ascending: Mechanism and Process in Physiological Adaptation to High-Altitude Hypoxia

To cope with the reduced availability of O2 at high altitude, air-breathing vertebrates have evolved myriad adjustments in the cardiorespiratory system to match tissue O2 delivery with metabolic O2 demand. We explain how changes at interacting steps of the O2 transport pathway contribute to plastic and evolved changes in whole-animal aerobic performance under hypoxia. In vertebrates native to high altitude, enhancements of aerobic performance under hypoxia are attributable to a combination of environ- mentally induced and evolved changes in multiple steps of the pathway. Additionally, evidence suggests that many high-altitude natives have evolved mechanisms for attenuating maladaptive acclimatization responses to hypoxia, resulting in counter-gradient patterns of altitudinal variation for key physiological phenotypes. For traits that exhibit counteracting environmental and genetic effects, evolved changes in phenotype may be cryptic under field conditions and can only be revealed by rearing representatives of high- and low-altitude populations under standardized environmental conditions to control for plasticity

\u3ci\u3eCynopterus sphinx\u3c/i\u3e

Order Chiroptera, Family Pteropodidae, Subfamily Pteropodinae, Tribe Cynopterini, Subtribe Cynopterina, Genus Cynopterus. Five species are recognized: C. brachyotis, C. horsfieldi, C. nusatenggara, C. sphinx, and C. titthaecheileus (Koopman, 1993). A key to the species is given in Lekagul and McNeely (1977)

The α\u3csup\u3eD\u3c/sup\u3e-Globin Gene Originated via Duplication of an Embryonic α-Like Globin Gene in the Ancestor of Tetrapod Vertebrates

Gene duplication is thought to play an important role in the co-option of existing protein functions to new physiological pathways. The globin superfamily of genes provides an excellent example of the kind of physiological versatility that can be attained through the functional and regulatory divergence of duplicated genes that encode different subunit polypeptides of the tetrameric hemoglobin protein. In contrast to prevailing views about the evolutionary history of the α-globin gene family, here we present phylogenetic evidence that the αA- and αD-globin genes are not the product of a single, tandem duplication of an ancestral globin gene with adult function in the common ancestor of extant birds, reptiles, and mammals. Instead, our analysis reveals that the αD-globin gene of amniote vertebrates arose via duplication of an embryonic α-like globin gene that predated the radiation of tetrapods. The important evolutionary implication is that the distinct biochemical properties of αD-hemoglobin (HbD) are not exclusively derived characters that can be attributed to a postduplication process of neofunctionalization. Rather, many of the distinct biochemical properties of HbD are retained ancestral characters that reflect the fact that the αD-globin gene arose via duplication of a gene that had a larval/embryonic function. These insights into the evolutionary origin of HbD illustrate how adaptive modifications of physiological pathways may result from the retention and opportunistic co-option of ancestral protein functions

New Genes Originated via Multiple Recombinational Pathways in the β-Globin Gene Family of Rodents

Species differences in the size or membership composition of multigene families can be attributed to lineage-specific additions of new genes via duplication, losses of genes via deletion or inactivation, and the creation of chimeric genes via domain shuffling or gene fusion. In principle, it should be possible to infer the recombinational pathways responsible for each of these different types of genomic change by conducting detailed comparative analyses of genomic sequence data. Here, we report an attempt to unravel the complex evolutionary history of the β-globin gene family in a taxonomically diverse set of rodent species. The main objectives were: 1) to characterize the genomic structure of the β-globin gene cluster of rodents; 2) to assign orthologous and paralogous relationships among duplicate copies of β-like globin genes; and 3) to infer the specific recombinational pathways responsible for gene duplications, gene deletions, and the creation of chimeric fusion genes. Results of our comparative genomic analyses revealed that variation in gene family size among rodent species is mainly attributable to the differential gain and loss of later expressed β-globin genes via unequal crossing-over. However, two distinct recombinational mechanisms were implicated in the creation of chimeric fusion genes. In muroid rodents, a chimeric γ/ε fusion gene was created by unequal crossing-over between the embryonic ε- and γ-globin genes. Interestingly, this γ/ε fusion gene was generated in the same fashion as the “anti-Lepore” 5’-δ-(β/δ)-β-3’ duplication mutant in humans (the reciprocal exchange product of the pathological hemoglobin Lepore deletion mutant). By contrast, in the house mouse, Mus musculus, a chimeric β/δ fusion pseudogene was created by a β-globin → δ-globin gene conversion event. Although the γ/ε and β/δ fusion genes share a similar chimeric gene structure, they originated via completely different recombinational pathways