Genetic evidence for widespread population size expansion in North American boreal birds prior to the Last Glacial Maximum.

Pleistocene climate cycles are well documented to have shaped contemporary species distributions and genetic diversity. Northward range expansions in response to deglaciation following the Last Glacial Maximum (LGM; approximately 21 000 years ago) are surmised to have led to population size expansions in terrestrial taxa and changes in seasonal migratory behaviour. Recent findings, however, suggest that some northern temperate populations may have been more stable than expected through the LGM. We modelled the demographic history of 19 co-distributed boreal-breeding North American bird species from full mitochondrial gene sets and species-specific molecular rates. We used these demographic reconstructions to test how species with different migratory strategies were affected by glacial cycles. Our results suggest that effective population sizes increased in response to Pleistocene deglaciation earlier than the LGM, whereas genetic diversity was maintained throughout the LGM despite shifts in geographical range. We conclude that glacial cycles prior to the LGM have most strongly shaped contemporary genetic diversity in these species. We did not find a relationship between historic population dynamics and migratory strategy, contributing to growing evidence that major switches in migratory strategy during the LGM are unnecessary to explain contemporary migratory patterns.

Disruptions of feather carotenoid pigmentation in a subset of hybrid northern flickers (Colaptes auratus) may be linked to genetic incompatibilities.

Hybridization can bring in single individuals alleles that were never designed to work together, which can result in unexpected or transgressive phenotypes. The Yellow-shafted (auratus group) and Red-shafted (cafer group) subspecies groups of the Northern Flicker (Colaptes auratus) differ conspicuously in the coloration of their flight feathers, but hybridize freely where their ranges overlap in western North America. The difference in color is largely the result of the Red-shafted form harboring ketolated products at C4(4') of the carotenoids found in the Yellow-shafted form. Characterizing the carotenoid pigments in a series of birds of intermediate color (presumed hybrids) revealed that most accumulated a product of ß-cryptoxanthin with a keto group on its hydroxylated ring (3-hydroxy-echinenone), while a few accumulated the product with a keto group on the unhydroxylated ring (3'-hydroxy-echinenone). Surprisingly, the latter group also had feather barbs that were noticeably yellower than the associated rachis, corresponding to a lower level of ketolation at C4(4'). We assessed possible biochemical explanations for the differences by probing the relative carotenoid concentration data in individuals of varying color. The difference between the hybrids could not be explained by the general level of ketolation of carotenoids or a particular selectivity of the 4-ketolase involved. We present a testable genetic explanation that invokes incompatibilities between divergent alleles of the two parental forms at interacting loci. Because the idiosyncrasies affect oxidation, they may be the product of mitonuclear incompatibilities.

Plumage pigment differences underlying the yellow-red differentiation in the Northern Flicker (Colaptes auratus).

Elucidating the processes that create species differences is a central goal of evolutionary biology. The Northern Flicker (Colaptes auratus) exists as two well-differentiated subspecies groups in North America, the Yellow-shafted (auratus group) and Red-shafted Flickers (cafer group), which differ strikingly in the color of the underside and rachises of flight feathers, and of malar and nuchal patches. We investigated the physiological basis of these conspicuous phenotypic differences by identifying and quantifying the pigments involved. The yellow feathers of auratus contained carotenoids commonly found in nature (lutein, ß-cryptoxanthin, zeaxanthin and ß-carotene). The orange to red shafts/vanes of cafer and hybrids contained these carotenoids as well as mono- and diketo-carotenoids (notably adonirubin, α-doradexanthin, canthaxanthin, astaxanthin), representing oxygenated products at carbon C4(4') of the carotenoids present in auratus. Oxygenation of feather carotenoids at C4(4') correlated closely with shaft/vane redness. Carotenoid hydroxylation at C3(3') and the proportion of carotenoids with ε end-rings also varied with color and belie differences in the activity of several carotenoid-modifying enzymes between the two subspecies groups. Curiously, occasional yellow feathers in red-shafted individuals had the carotenoids of auratus, hence the differences are not constitutive in cafer, underscoring regulatory differences. The red malar stripe of cafer, the black malar stripe and red nuchal patch of auratus all contained similar types and amounts of carotenoids, mostly 3-hydroxy-4-keto-carotenoids. The biochemical differences between two strongly differentiated forms we uncovered shed light on how plumage coloration can change over evolutionary time and point to further avenues of research.

Plumage carotenoids of the Pin-tailed Manakin (Ilicura militaris): evidence for the endogenous production of rhodoxanthin from a colour variant.

The Pin-tailed Manakin (Ilicura militaris) is a small, sexually dimorphic, frugivorous suboscine songbird (Pipridae; Passeriformes; Aves) endemic to the Atlantic Forest of Brazil. A variant individual of this species was recently described in which the red patches that characterise the male's Definitive plumage were replaced by orange-yellow ones. We show here that the pigments in the feathers of the colour variant are common dietary carotenoids (zeaxanthin, beta-cryptoxanthin), not carotenoids synthesised by birds, lending support to the suggestion that the individual is a colour mutant lacking the capability to transform yellow dietary pigments into the red pigments normally present in these feathers. By comparison, the yellow crown feathers of a close relative, the Golden-winged Manakin (Masius chrysopterus), contained predominantly endogenously produced epsilon-caroten-3'-ones. Surprisingly, the normal-coloured feathers of the male Pin-tailed Manakin owe their red hue to rhodoxanthin, an unusual carotenoid more commonly found in plants, rather than 4-keto-carotenoids typically found in red plumages and found lacking in previously characterised bird colour variants. The implication is that birds, like the tilapia fish, may be able to synthesise this unusual pigment endogenously from dietary precursors. A newly described carotenoid, 6-hydroxy-epsilon,epsilon-carotene-3,3'-dione, here named piprixanthin, present in the red feathers of the Pin-tailed Manakin, provides a plausible intermediate between epsilon,epsilon-carotene-3,3'-dione (canary-xanthophyll B), a bright yellow pigment found in this and other songbirds, and rhodoxanthin. It is apparent that pigeons (Columbidae, Columbiformes) also have the capability to produce rhodoxanthin, and a structurally related pigment, endogenously. The ability to synthesise rhodoxanthin might have arisen at least twice in birds.

Countergradient variation in the sexual coloration of guppies (Poecilia reticulata): drosopterin synthesis balances carotenoid availability.

Trinidad guppies (Poecilia reticulata) are distributed along an environmental gradient in carotenoid availability that limits the carotenoid content of the orange spots of males. The amount of synthetic red pteridines (drosopterins) in the orange spots covaries with the carotenoid content, such that the ratio of the two types of pigments is roughly conserved across streams. Carotenoids and drosopterins have different spectral properties and thus the ratio of the two types of pigments affects the shape of the orange spot reflectance spectrum. Geographic conservation of the carotenoid:drosopterin ratio suggests that males may be under selection to maintain a particular hue. We tested this hypothesis by comparing the pigmentation and coloration of guppies from six streams in the field to that of second-generation descendants of the same populations raised on three dietary carotenoid levels in the laboratory. The results show clearly that the geographic variation in drosopterin production is largely genetic and that the hue of the orange spots is conserved among populations in the field, relative to the laboratory diet groups. This is a countergradient pattern because genetic differences between populations in drosopterin production mask the effect of carotenoid availability on the hue of the orange spots. The potential for countergradient sexual selection to contribute to reproductive isolation between populations is discussed.

Marginal differentiation between the sexual and general carotenoid pigmentation of guppies (Poecilia reticulata) and a possible visual explanation.

We present the first detailed analysis of carotenoid pigmentation of the integument of guppies (Poecilia reticulata Peters), quantifying variation in carotenoid content and composition of wild guppies from three drainages on Trinidad (1) between the sexual and general pigmentation of males, (2) between the sexes, and (3) geographically in relation to carotenoid availability. We report that the carotenoid pigments in the integument of guppies are predominantly esters of tunaxanthin. The peak wavelength of carotenoids in the orange spots of males lay only ca. 2.8 nm higher than that of pigments outside of the orange spots, and the peak wavelength of carotenoids in the male whole integument does not differ from that in the female whole integument. Carotenoid composition of the general integument of females and the non-orange spot fraction of males, but not of the orange spot fraction of males, varied with diet, correlating with the ratio beta-carotene to lutein in the different streams. Male guppies deposit higher concentrations of carotenoids in their orange spots than in the rest of the integument (five to nine times higher), but not at the expense of the general integument, which was similarly endowed as the general integument of females, even in carotenoid-poor streams. Presumably males absorb/retain more pigments than females. Photoreceptor-based simulations suggest that tunaxanthin provides both greater brightness and chroma than would 4-keto-carotenoids such as astaxanthin.