Primary Selective Force Research Articles

Genomic exploration and functional insights into the diversification of the Snf2 gene family in subgenomes of Triticum aestivum

Sucrose nonfermenting 2 (Snf2) family proteins function as the ATP-dependent catalytic engines of chromatin remodeling complexes, which harness ATP hydrolysis energy to alter chromatin structure and nucleosome positioning, enabling regulatory factor access to DNA. Plant genomes contain numerous Snf2 family proteins, several of which have been demonstrated to act as key developmental regulators at different stages in model plants like Arabidopsis and rice. Despite their vital roles, the Snf2 genes in Triticum aestivum remain largely uncharacterized. Here, we report the identification of 112 wheat Snf2 genes that were unevenly distributed across the 21 chromosomes, with 40 genes on the A subgenome, 33 on the B subgenome, and 39 on the D subgenome, and phylogenetically classified these Snf2 genes into 18 subfamilies related to the 6 Snf2 groups in Arabidopsis. Evolutionary analysis revealed that purifying selection has largely driven the evolution of Snf2 genes, acting as the primary selective force shaping the Snf2 gene family in wheat, while segmental duplications have served as the main mechanism for expanding the gene family. All identified Snf2 proteins contained at least one Helicase_C and SNF2_N domain among 10 conserved domains, and their gene structures consisted of 3–38 exons. Tissue-specific expression analysis uncovered distinct expression patterns among Snf2 gene family members, including some with enhanced reproductive tissue expression, while analysis under various abiotic and biotic stresses revealed differential regulation of specific family members in response to these conditions. Overall, these systematic analyses including identification, evolutionary relationships, and expression profiling provide valuable insights into the wheat Snf2 family while establishing a genomic framework to elucidate Snf2 functional roles in wheat growth, development, and stress responses.

New tools suggest a middle Jurassic origin for mammalian endothermy: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time.

We suggest that mammalian endothermy was established amongst Middle Jurassic crown mammals, through reviewing state-of-the-art fossil and living mammal studies. This is considerably later than the prevailing paradigm, and has important ramifications for the causes, pattern, and pace of physiological evolution amongst synapsids. Most hypotheses argue that selection for either enhanced aerobic activity, or thermoregulation was the primary driver for synapsid physiological evolution, based on a range of fossil characters that have been linked to endothermy. We argue that, rather than either alternative being the primary selective force for the entirety of endothermic evolution, these characters evolved quite independently through time, and across the mammal family tree, principally as a response to shifting environmental pressures and ecological opportunities. Our interpretations can be tested using closely linked proxies for both factors, derived from study of fossils of a range of Jurassic and Cretaceous mammaliaforms and early mammals.

Cotton Leafroll Dwarf Virus US Genomes Comprise Divergent Subpopulations and Harbor Extensive Variability.

Cotton leafroll dwarf virus (CLRDV) was first reported in the United States (US) in 2017 from cotton plants in Alabama (AL) and has become widespread in cotton-growing states of the southern US. To investigate the genomic variability among CLRDV isolates in the US, complete genomes of the virus were obtained from infected cotton plants displaying mild to severe symptoms from AL, Florida, and Texas. Eight CLRDV genomes were determined, ranging in size from 5865 to 5867 bp, and shared highest nucleotide identity with other CLRDV isolates in the US, at 95.9–98.7%. Open reading frame (ORF) 0, encoding the P0 silencing suppressor, was the most variable gene, sharing 88.5–99.6% and 81.2–89.3% amino acid similarity with CLRDV isolates reported in cotton growing states in the US and in Argentina and Brazil in South America, respectively. Based on Bayesian analysis, the complete CLRDV genomes from cotton in the US formed a monophyletic group comprising three relatively divergent sister clades, whereas CLRDV genotypes from South America clustered as closely related sister-groups, separate from US isolates, patterns reminiscent of phylogeographical structuring. The CLRDV isolates exhibited a complex pattern of recombination, with most breakpoints evident in ORFs 2 and 3, and ORF5. Despite extensive nucleotide diversity among all available CLRDV genomes, purifying selection (dN/dS < 1) was implicated as the primary selective force acting on viral protein evolution.

Sperm quality and quantity evolve through different selective processes in the Phasianidae

Sperm competition is often considered the primary selective force underlying the rapid and diversifying evolution of ejaculate traits. Yet, several recent studies have drawn attention to other forms of selection with the potential of exceeding the effects of sperm competition. Since ejaculates are complex, multivariate traits, it seems plausible that different ejaculate components vary in their responses to different selective pressures. Such information, however, is generally lacking as individual ejaculate traits tend to be studied in isolation. Here, we studied the macroevolutionary patterns of ejaculate volume, sperm number, sperm length and the proportion of viable normal sperm in response to varying levels of sperm competition, body size and the duration of female sperm storage in pheasants and allies (Phasianidae). Ejaculate volume, sperm number and sperm viability were all relatively higher in polygamous than in monogamous mating systems. However, whereas ejaculate volume additionally covaried with body size, sperm number instead increased with the female sperm-storage duration, in conjunction with a decrease in sperm length. Overall, our results revealed important details on how different forms of selection can jointly shape ejaculates as complex, composite traits.

Protoconch enlargement in Western Atlantic turritelline gastropod species following the closure of the Central American Seaway.

The closure of the late Neogene interoceanic seaways between the Western Atlantic (WA) and Tropical Eastern Pacific (TEP)—commonly referred to as the Central American Seaway—significantly decreased nutrient supply in the WA compared to the TEP. In marine invertebrates, an increase in parental investment is expected to be selectively favored in nutrient‐poor marine environments as prolonged feeding in the plankton becomes less reliable. Here, we examine turritelline gastropods, which were abundant and diverse across this region during the Neogene and serve as important paleoenvironmental proxies, and test whether species exhibit decreased planktotrophy in the WA postclosure as compared to preclosure fossils and extant TEP species. We also test for differences in degree of planktotrophy in extant sister species pairs. Degree of planktotrophy was inferred by measuring the size of protoconchs, the species' larval shell that represents egg size. Protoconch size was compared between extant postclosure WA and TEP species and preclosure fossil species. To compare extant sister species, we reconstructed the phylogeny of available WA and TEP species using one nuclear (H3) and three mitochondrial markers (12S, 16S, and COI). Compared to the preclosure fossils, protoconch size increased in WA species but remained the same in the TEP species. In the two extant sister species pairs recovered in the phylogenetic analysis, the WA species are inferred to be nonplanktotrophic while the TEP species are planktotrophic. This suggests that decreased nutrient availability and primary productivity in the WA may have driven this change in developmental mode, and was the primary selective force resulting in postclosure turritelline extinctions.

The spread of resistance to imidacloprid is restricted by thermotolerance in natural populations of Drosophila melanogaster.

Imidacloprid, the world's most used insecticide, has caused considerable controversy due to harmful effects on non-pest species and increasing evidence showing that insecticides have become the primary selective force in many insect species. The genetic response to insecticides is heterogeneous across populations and environments, leading to more complex patterns of genetic variation than previously thought. This motivated the investigation of imidacloprid resistance at different temperatures in natural populations of Drosophila melanogaster originating from four climate extremes replicated across two continents. Population and quantitative genomic analysis, supported by functional tests, have revealed a mixed genetic architecture to resistance involving major genes (Paramyosin and Nicotinic-Acetylcholine Receptor Alpha 3) and polygenes with a major trade-off with thermotolerance. Reduced genetic differentiation at resistance-associated loci indicated enhanced gene flow at these loci. Resistance alleles showed stronger evidence of positive selection in temperate populations compared to tropical populations in which chromosomal inversions In(2 L)t, In(3 R)Mo and In(3 R)Payne harbour susceptibility alleles. Polygenic architecture and ecological factors should be considered when developing sustainable management strategies for both pest and beneficial insects.

Importance of considering soils in seed transfer zone development: evidence from a study of the native Bromus marginatus.

Seed transfer zones, which define the geographical relationship between adaptive traits and environmental factors, are increasingly used to determine the source populations that can be combined in restoration and revegetation. Climatic variables have been the most commonly used environmental data in transfer zone development, even though soils are also a primary selective force on plants. We assessed the importance of including soils in seed transfer zones using Bromus marginatus, a native grass used for restoration and revegetation in the western United States, as an example. Seeds were collected from 64 populations across Montana and Idaho and grown in a common garden for two years. We assessed among-population variation based on 11 traits related to germination rate, plant size, vigor, inflorescence number, survival, and carbon isotope discrimination (∆13 ), and used this variation to develop seed transfer zone maps using two approaches: (1) a conventional approach, using only climatic variables (climate only) and (2) an expanded approach that included soils and climatic variables (soils+climate). The most influential drivers of trait variation were factors related to soil water availability: soil order, available water content (AWC), and organic carbon levels. Populations from areas with andic soils, which have high soil AWC and soil organic carbon, had low germination, limited first-year survival, low ∆13 , and small seeds. Growing season length and winter temperatures were also predictive of trait variation. In comparison to climate-only models, soils+climate models explained 11% more variance (120% relative increase) for ∆13 and an average of 4.5% more (27% relative increase) for growth traits and survival. The transfer zone map developed using soils+climate differed from the climate-only map in both spatial pattern of ecotypic variation and number of transfer zones; the soils+climate map had more zones and a higher proportion of small (<4km2 ) transfer zone patches, while the climate-only map had more large patches >37km2 . Including soils in transfer zone development may identify adaptive trait variation that is obscured by large-scale differences in climate and could improve plant materials used for ecosystem management.

Cellular allorecognition and its roles in Dictyostelium development and social evolution.

The social amoeba Dictyostelium discoideum is a tractable model organism to study cellular allorecognition, which is the ability of a cell to distinguish itself and its genetically similar relatives from more distantly related organisms. Cellular allorecognition is ubiquitous across the tree of life and affects many biological processes. Depending on the biological context, these versatile systems operate both within and between individual organisms, and both promote and constrain functional heterogeneity. Some of the most notable allorecognition systems mediate neural self-avoidance in flies and adaptive immunity in vertebrates. D. discoideum's allorecognition system shares several structures and functions with other allorecognition systems. Structurally, its key regulators reside at a single genomic locus that encodes two highly polymorphic proteins, a transmembrane ligand called TgrC1 and its receptor TgrB1. These proteins exhibit isoform-specific, heterophilic binding across cells. Functionally, this interaction determines the extent to which co-developing D. discoideum strains co-aggregate or segregate during the aggregation phase of multicellular development. The allorecognition system thus affects both development and social evolution, as available evidence suggests that the threat of developmental cheating represents a primary selective force acting on it. Other significant characteristics that may inform the study of allorecognition in general include that D. discoideum's allorecognition system is a continuous and inclusive trait, it is pleiotropic, and it is temporally regulated.

Feeding ecology is the primary driver of beak shape diversification in waterfowl

Summary The diversity of beak shapes among birds is often assumed to be largely the result of adaptations to different feeding behaviors and diets. However, this assumption has only been tested for a small subset of avian diversity, primarily within the order Passeriformes. Moreover, given the role of the beak in behaviors other than feeding and given that most previously identified beak‐feeding associations concern beak size rather than shape, it remains unclear how much of beak shape diversity is explained by feeding ecology and what functional explanations account for these differences in shape. I quantified the association between beak shape and feeding ecology for 42 species in the bird order Anseriformes (waterfowl) using 3D curvature of the upper beak collected from museum specimens and continuous dietary data compiled from the literature. I also tested whether leverage or stress resistance of the beak explains the association between beak shape and feeding ecology. Diet is strongly and significantly correlated with beak shape in waterfowl. An ancestral beak shape reconstruction and the reconstructed diet of the anseriform fossil Presbyornis both support filter‐feeding as ancestral for most waterfowl, followed by multiple, significantly convergent transitions from a duck‐like beak toward a more goose‐like beak. The evolution of a more goose‐like beak is associated with increased consumption of leaves, decreased consumption of invertebrates, and an increase in mechanical advantage of the beak. Moreover, no association was identified between size (measured as either beak size or body mass) and feeding ecology nor between size and beak shape. These results demonstrate that feeding ecology has acted as the primary selective force in the diversification of waterfowl beak shapes, including the convergent originations of geese. Thus, rapid and convergent adaptation of the beak to feeding is not limited to passerines nor is it limited to size‐correlated shape changes. The positive evolutionary correlation between mechanical advantage and herbivory shows that lever mechanics can explain the functional evolution of the kinetic upper beak in birds. These results also suggest that functions of the beak other than feeding may play a minor role in explaining overall beak shape diversity. A lay summary is available for this article.

Unifying Within- and Between-Generation Bet-Hedging Theories: An Ode to J. H. Gillespie.

In the 1970s, John Gillespie introduced two principles in which evolution selects for genotypes with lower variation in offspring numbers. First, if the variation in offspring number primarily occurs within generations, the strength of this selective force is inversely proportional to population size. Second, if this variation primarily occurs between generations, the strength of this selective force is proportional to the variance and independent of population size. These principles lie at the core of bet-hedging theory. Using the common currency of fixation probabilities, I derive a general principle for which within-generation correlation of individual fitness acts as a dial between Gillespie's limiting cases. At low correlations, within-generation variation is the primary selective force. At high correlations, between-generation variation is the dominant selective force. As corollary of this general principle, selection for diversified bet-hedging strategies is shown to require higher within-generation environmental correlations in smaller populations.

Reproductive mode evolution in lizards revisited: updated analyses examining geographic, climatic and phylogenetic effects support the cold-climate hypothesis.

Viviparity, the bearing of live young, has evolved well over 100 times among squamate reptiles. This reproductive strategy is hypothesized to allow maternal control of the foetus' thermal environment and thereby to increase the fitness of the parents and offspring. Two hypotheses have been posited to explain this phenomenon: (i) the cold-climate hypothesis (CCH), which advocates low temperatures as the primary selective force; and (ii) the maternal manipulation hypothesis (MMH), which advocates temperature variability as the primary selective force. Here, we investigate whether climatic and geographic variables associated with the CCH vs. the MMH best explain the current geographical distributions of viviparity in lizards while incorporating recent advances in comparative methods, squamate phylogenetics and geospatial analysis. To do this, we compared nonphylogenetic and phylogenetic models predicting viviparity based on point-of-capture data from 20,994 museum specimens representing 215 lizard species in conjunction with spatially explicit bioclimatic and geographic (elevation and latitude) data layers. The database we analysed emphasized Nearctic lizards from three species-rich genera (Phrynosoma, Plestiodon and Sceloporus); however, we additionally analysed a less substantial, but worldwide sample of species to verify the universality of our Nearctic results. We found that maximum temperature of the warmest month (and, less commonly, elevation and maximum temperature of the driest quarter) was frequently the best predictor of viviparity and showed an association consistent with the CCH. Our results strongly favour the CCH over the MMH in explaining lizard reproductive mode evolution.

Rising costs of care make spiny chromis discerning parents

When the costs of parental care do not scale with the number of offspring being cared for, inclusion of non-descendant young into broods can be advantageous, leading to systems of alloparental care. However, if the cost of care scales with the number of offspring, selection may act against misdirected parental care. The spiny chromis, Acanthochromis polyacanthus, is a marine fish with extended biparental care, and broods that increase in size over the care period strongly suggest that alloparental care occurs in this species. However, A. polyacanthus parents directly provision their offspring by producing ectodermal mucus for their fry to feed on. The costs of such provisioning may scale with brood size, potentially increasing the costs of parental care. Using wild A. polyacanthus pairs, we tested whether foreign offspring are accepted into established broods, and measured how brood defence effort and mucal feeding scale with brood size. We found that A. polyacanthus discriminate between their own and foreign young, vigorously expelling experimentally introduced foreign offspring. Although defensive effort did not scale with brood size, mucal feeding was strongly dependent on brood size, and this increasing cost of care likely acts as the primary selective force on parental discrimination and rejection of foreign fry in A. polyacanthus.

Evolution of fruit traits in ficus subgenus Sycomorus (Moraceae): to what extent do frugivores determine seed dispersal mode?

Fig trees are a ubiquitous component of tropical rain forests and exhibit an enormous diversity of ecologies. Focusing on Ficus subgenus Sycomorus, a phenotypically diverse and ecologically important Old World lineage, we examined the evolution of fruit traits using a molecular phylogeny constructed using 5 kilobases of DNA sequence data from 63 species (50% of global diversity). In particular, we ask whether patterns of trait correlations are consistent with dispersal agents as the primary selective force shaping morphological diversity or if other ecological factors may provide a better explanation? Fig colour, size and placement (axial, cauliflorous, or geocarpic) were all highly evolutionarily liable, and the same fruit traits have evolved in different biogeographic regions with substantially different dispersal agents. After controlling for phylogenetic autocorrelation, we found that fig colour and size were significantly associated with fig placement and plant-life history traits (maximum plant height and leaf area, respectively). However, contrary to prevailing assumptions, fig placement correlated poorly with known dispersal agents and appears more likely determined by other factors, such as flowering phenology, nutrient economy, and habitat preference. Thus, plant life-history, both directly and through its influence on fig placement, appears to have played a prominent role in determining fruit traits in these figs.

The probability of the improbable: society–nature coevolution

.This article aims to show how evolutionary theory, social‐metabolism and sociological systems theory can be utilized to develop a concept of society–nature coevolution. The article begins with a conception of industrialization as a socio‐metabolic transition, that is, a major transformation in the energetic and consequently material basis of society. This transition to industrial metabolism was essential for the emergence and maintenance of industrial societies and is at the same time the main cause of global environmental change. The article proceeds by asking what the notion of society–nature coevolution can potentially contribute to understanding environmental sustainability problems. An elaborated concept of coevolution hinges on (1) a more precise and sociologically more meaningful concept of cultural evolution and (2) understanding how cultural evolution is linked to the environment. Next I briefly outline major lines of thought and controversies surrounding the idea of cultural evolution. The direction proposed here commences with an abstract version of Darwinian evolution, which is then re‐specified for social systems, understood as communication systems, as developed by Luhmann. The re‐specification implies three important changes in the theoretical outline of cultural evolution: first, shifting from the human population to the communication system as the unit of cultural evolution and to single communications as the unit of cultural variation; second, shifting from transmission or inheritance to reproduction as necessary condition for evolution; and third, shifting from purely internal (communicative) forces of selection towards including also environmental selection. Adopting elements from the work of Hägerstrand and Boserup, the primary environmental selective force in cultural evolution is conceptualized as the historically variable constraints in human time–space occupation. In the conclusions I tie the argument back to its beginning, by arguing that the most radical changes in human time–space occupation have been enabled by major socio‐metabolic transitions in the energy system.

Absence of positive selection on centromeric histones in Tetrahymena suggests unsuppressed centromere: drive in lineages lacking male meiosis.

Centromere-drive is a process where centromeres compete for transmission through asymmetric "female" meiosis for inclusion into the oocyte. In symmetric "male" meiosis, all meiotic products form viable germ cells. Therefore, the primary incentive for centromere-drive, a potential transmission bias, is believed to be missing from male meiosis. In this article, we consider whether male meiosis also bears the primary cost of centromere-drive. Because different taxa carry out different combinations of meiotic programs (symmetric + asymmetric, symmetric only, asymmetric only), it is possible to consider the evolutionary consequences of centromere-drive in the context of these differing systems. Groups with both types of meiosis have large, rapidly evolving centromeric regions, and their centromeric histones (CenH3s) have been shown to evolve under positive selection, suggesting roles as suppressors of centromere-drive. In contrast, taxa with only symmetric male meiosis have shown no evidence of positive selection in their centromeric histones. In this article, we present the first evolutionary analysis of centromeric histones in ciliated protozoans, a group that only undergoes asymmetric "female" meiosis. We find no evidence of positive selection acting on CNA1, the CenH3 of Tetrahymena species. Cytological observations of a panel of Tetrahymena species are consistent with dynamic karyotype evolution in this lineage. Our findings suggest that defects in male meiosis, and not mitosis or female meiosis, are the primary selective force behind centromere-drive suppression. Our study raises the possibility that taxa like ciliates, with only female meiosis, may therefore undergo unsuppressed centromere drive.Electronic supplementary materialThe online version of this article (doi:10.1007/s00239-011-9449-0) contains supplementary material, which is available to authorized users.

Predictors of orbital convergence in primates: A test of the snake detection hypothesis of primate evolution

Traditional explanations for the evolution of high orbital convergence and stereoscopic vision in primates have focused on how stereopsis might have aided early primates in foraging or locomoting in an arboreal environment. It has recently been suggested that predation risk by constricting snakes was the selective force that favored the evolution of orbital convergence in early primates, and that later exposure to venomous snakes favored further degrees of convergence in anthropoid primates. Our study tests this snake detection hypothesis (SDH) by examining whether orbital convergence among extant primates is indeed associated with the shared evolutionary history with snakes or the risk that snakes pose for a given species. We predicted that orbital convergence would be higher in species that: 1) have a longer history of sympatry with venomous snakes, 2) are likely to encounter snakes more frequently, 3) are less able to detect or deter snakes due to group size effects, and 4) are more likely to be preyed upon by snakes. Results based on phylogenetically independent contrasts do not support the SDH. Orbital convergence shows no relationship to the shared history with venomous snakes, likelihood of encountering snakes, or group size. Moreover, those species less likely to be targeted as prey by snakes show significantly higher values of orbital convergence. Although an improved ability to detect camouflaged snakes, along with other cryptic stimuli, is likely a consequence of increased orbital convergence, this was unlikely to have been the primary selective force favoring the evolution of stereoscopic vision in primates.

Comparing Phylogeny and the Predicted Pathogenicity of Protein Variations Reveals Equal Purifying Selection across the Global Human mtDNA Diversity

We used detailed phylogenetic trees for human mtDNA, combined with pathogenicity predictions for each amino acid change, to evaluate selection on mtDNA-encoded protein variants. Protein variants with high pathogenicity scores were significantly rarer in the older branches of the tree. Variants that have formed and survived multiple times in the human phylogenetics tree had significantly lower pathogenicity scores than those that only appear once in the tree. We compared the distribution of pathogenicity scores observed on the human phylogenetic tree to the distribution of all possible protein variations to define a measure of the effect of selection on these protein variations. The measured effect of selection increased exponentially with increasing pathogenicity score. We found no measurable difference in this measure of purifying selection in mtDNA across the global population, represented by the macrohaplogroups L, M, and N. We provide a list of all possible single amino acid variations for the human mtDNA-encoded proteins with their predicted pathogenicity scores and our measured selection effect as a tool for assessing novel protein variations that are often reported in patients with mitochondrial disease of unknown origin or for assessing somatic mutations acquired through aging or detected in tumors.

Effect of Correlated tRNA Abundances on Translation Errors and Evolution of Codon Usage Bias

Despite the fact that tRNA abundances are thought to play a major role in determining translation error rates, their distribution across the genetic code and the resulting implications have received little attention. In general, studies of codon usage bias (CUB) assume that codons with higher tRNA abundance have lower missense error rates. Using a model of protein translation based on tRNA competition and intra-ribosomal kinetics, we show that this assumption can be violated when tRNA abundances are positively correlated across the genetic code. Examining the distribution of tRNA abundances across 73 bacterial genomes from 20 different genera, we find a consistent positive correlation between tRNA abundances across the genetic code. This work challenges one of the fundamental assumptions made in over 30 years of research on CUB that codons with higher tRNA abundances have lower missense error rates and that missense errors are the primary selective force responsible for CUB.

Predation as the primary selective force in recurrent evolution of gigantism in Poecilozonites land snails in Quaternary Bermuda

During the last half million years, pulses of gigantism in the anagenetic lineage of land snails of the subgenus Poecilozonites on Bermuda were correlated with glacial periods when lower sea level resulted in an island nearly an order of magnitude larger than at present. During those periods, the island was colonized by large vertebrate predators that created selection pressure for large size and rapid growth in the snails. Extreme reduction in land area from rising seas, along with changes in ecological conditions at the onset of interglacial episodes, marked extinction events for large predators, after which snails reverted to much smaller size. The giant snails were identical in morphology during the last two glacials when the predators included a large flightless rail Rallus recessus (marine isotope stages (MIS) 4-2) and a crane Grus latipes and a duck Anas pachysceles (MIS 6). In a preceding glacial period (MIS 10), when the fauna also included the tortoise Hesperotestudo bermudae, the snails were not only large, but the shells were much thicker, presumably to prevent crushing by tortoises. Evolution of Poecilozonites provides an outstanding example of dramatic morphological change in response to environmental pressures in the absence of cladogenesis.

Short communication: Absence of protandry in a population of Gray Catbirds Dumetella carolinensis

Sex-based differential arrival on the breeding grounds at the completion of migration has been poorly studied in monochromatic and monomorphic species because individuals are difficult to sex without recapture or molecular tests. We examined sex-based arrival timing to the breeding grounds of the Gray Catbird Dumetella carolinensis, a monochromatic species. We also examined differences in size and arrival by age as these are well documented to affect arrival timing. Older birds were larger than younger birds, and older males were larger than older females. Older birds arrived back on the breeding grounds earlier than younger birds, as documented in many species, but males did not precede females. Our results provide evidence of sexual size dimorphism in Catbirds and add to a growing body of information on arrival timing in monochromatic species. Differences in intensity of sex-based competition for breeding resources are hypothesized as the primary selective force for one sex to precede the other to the breeding grounds (Kokko 1999, Kokko et al. 2006). Typically, males arrive at the breeding grounds in advance of females (Morbey & Ydenberg 2001), a phenomenon known as protandry (Rubolini et al. 2004). This is because males are thought to experience more intensive selective pressure for acquiring territories or other resources for attracting a mate than females (but see Kokko et al. 2006 for a discussion of why females may also be under selection pressure for earlier arrival to compete for resources or mates). Furthermore, comparative studies suggest that the extent of sexual selection experienced by males, as evidenced by the degree of sexual dichromatism (Moller & Birkhead 1994, Owens & Hartley 1998), sexual size dimorphism (Owens & Hartley 1998, Dunn et al. 2001), polygyny or rate of extra-pair paternity (Coppack et al. 2006), affects the degree of protandry. While protandry is well documented in dimorphic species (Rubolini et al. 2004), little is known about arrival dates by sex in monomorphic or monochromatic species (Rubolini et al. 2004, Edwards & Forbes 2007, Newton 2008). The primary reason for this is the difficulty in sexing individuals upon arrival at the migratory destination. Moreover, the limited studies to date have produced equivocal results. For example, male Spotted Flycatchers Muscicapa striata preceded females during passage on the island of Ventotene, Italy, (Mabey 2002), and male White-throated Sparrows Zonotrichia albicollis and Least Flycatchers Empidonax minimus preceded females during spring migration at Long Point Observatory, Ontario, Canada (Mills 2005). In contrast, Edwards and Forbes (2007) presented evidence that Song Sparrows Melospiza melodia did not exhibit protandry upon arrival at breeding grounds in Ontario. Our research on arrival and breeding in the Gray Catbird afforded us the opportunity to assess arrival timing by sex and age in a songbird migrant described as monochromatic and monomorphic (Cimprich & Moore 1995). One advantage of our study is that we captured birds both during the arrival period and throughout the breeding season, permitting us to determine an individual’s sex and relate it back to when it arrived at breeding grounds in northeastern Pennsylvania. Although there are several hypotheses regarding differences (or lack of differences) in arrival timing between the sexes (Morbey & Ydenberg 2001) here we focus on assessing whether catbirds are indeed monomorphic and whether they exhibit protandry.