Biology subjects

Abzhanov, A.

Publications and source records attributed to Abzhanov, A..

6 recordsLinked to original sources

bioRxiv2023

Beak morphometry and morphogenesis across avian radiations

Adaptive avian radiations associated with the diversification of bird beaks into a multitude of forms enabling different functions are exemplified by Darwins finches and Hawaiian honeycreepers. To elucidate the nature of these radiations, we quantified beak shape and skull shape using a variety of geometric measures that allowed us to collapse the variability of beak shape into a minimal set of geometric parameters. Furthermore, we find that just two measures of beak shape, the ratio of the width to length and the normalized sharpening rate (increase in the transverse beak curvature near the tip relative to that at the base of the beak), are both strongly correlated with diet, and thus indicative of how bite forces are correlated with beak shape. Finally, by considering how transverse sections to the beak centerline evolve with distance from the tip, we show that a simple geometry-driven growth law termed "modified mean curvature flow" captures the beak shapes of Darwins finches and Hawaiian honeycreepers. A surprising consequence of the simple growth law is that beak shapes that are not allowed based on the developmental program of the beak are also not observed in nature, suggesting a link between evolutionary morphology and development in terms of growth-driven developmental constraints.

evolutionary biology↗

bioRxiv2021

Darwin's finches - an adaptive radiation constructed from ancestral genetic modules

Recent adaptive radiations are models for investigating mechanisms contributing to the evolution of biodiversity. An unresolved question is the relative importance of new mutations, ancestral variants, and introgressive hybridization for phenotypic evolution and speciation. Here we address this issue using Darwins finches, which vary in size from an 8g warbler finch with a pointed beak to a 40g large ground finch with a massive blunt beak. We present a highly contiguous genome assembly for one of the species and investigate the genomic architecture underlying phenotypic diversity in the entire radiation. Admixture mapping for beak and body size in the small, medium and large ground finches revealed 28 loci showing strong genetic differentiation. These loci represent ancestral haplotype blocks with origins as old as the Darwins finch phylogeny (1-2 million years). Genes expressed in the developing beak are overrepresented in these genomic regions. Frequencies of allelic variants at the 28 loci covary with phenotypic similarities in body and beak size across the Darwins finch phylogeny. These ancestral haplotypes constitute genetic modules for selection, and act as key determinants of the exceptional phenotypic diversity of Darwins finches. Such ancestral haplotype blocks can be critical for how species adapt to environmental variability and change.

evolutionary biology↗

bioRxiv2021

BMP signaling underlies the craniofacial heterochrony in phyllostomid bats, a hyperdiverse mammal group

The potential for variation and the capacity to evolve in response to ecological opportunity are important aspects of an adaptive radiation. Identifying the origin of phenotypic variation, in which natural selection might act upon, is a major goal of evolutionary developmental biology. The New World leaf-nosed bats (phyllostomids) are a textbook example of an adaptive radiation. Their cranial morphology is diverse along relative facial length, which is related to their diets. We previously used geometric morphometrics to reveal peramorphosis, a type of heterochrony, in the cranial evolution among phyllostomid bats. We then demonstrated that the mechanism of peramorphic diversity in phyllostomid rostrum length resulted from altered cellular proliferation. Here, we investigate the progenitors of the face, the cranial neural crest, and a key signaling pathway related to their proliferation and differentiation into mature tissues: the bone morphogenetic protein (BMP). With geometric morphometrics, immunofluorescence, and confocal imaging--in three phyllostomid species and one outgroup bat species--we show the molecular patterns that underlie the adaptive and innovative traits seen in phyllostomid bats. Then, with mouse genetics, we mimic the BMP molecular pattern observed in nectar feeding bats and recapitulate the elongated morphological variation in mice. Surprisingly, we also observe an expansion in the nose-tip of mice, akin to the expanding leaf-nose tissue in phyllostomid bats. These data, combined with the mouse genetics literature on BMP signaling, suggest the BMP developmental pathway plays a central role in shaping the craniofacial variation necessary for adaptation in bats. Further, we speculate that the BMP signaling pathway could underlie other bizarre facial phenotypes in mammals that are derived from frontonasal mesenchyme, such as the proboscis. Overall, this study combines a comparative framework to developmental data, with a genetic approach, to directly investigate the role of development on complex morphology.

evolutionary biology↗

bioRxiv2021

Transspecies beak color polymorphism in the Darwin's finch radiation

Carotenoid-based polymorphisms are widespread in populations of birds, fish, and reptiles1, but little is known of how they affect fitness and are maintained as species multiply2. We report a combined field and molecular-genetic investigation of a nestling beak color polymorphism in Darwins finches. Beaks are pink or yellow, and yellow is recessive3. Here we show that the polymorphism arose in the Galapagos approximately half a million years ago through a regulatory mutation in the BCO2 gene, and is shared by 14 descendant species. The frequency of the yellow genotype is associated with cactus flower abundance in cactus finches, and is altered by introgressive hybridization. The polymorphism is most likely a balanced polymorphism, maintained by ecological selection pressures associated with diet, and augmented by occasional interspecific introgression. Polymorphisms that are hidden as adults, as here, may contribute to evolutionary diversification in underappreciated ways in other systems.

evolutionary biology↗

bioRxiv2020

Sex identification in embryos and adults of Darwins finches

Darwins finches, endemic to the Galapagos and Cocos islands, are an iconic example of adaptive radiation and evolution under natural selection. Comparative genetic studies using embryos of Darwins finches have shed light on the possible evolutionary processes underlying the speciation of this clade. Molecular identification of the sex of embryonic samples is important for such studies, where this information often cannot be inferred otherwise. We tested a fast and simple chicken embryo protocol for extraction of genomic DNA on Darwins finch embryos. In addition, we suggest modifications to two of the previously reported PCR primer sets for CHD1, a gene used for sexing in adult passerine birds. The sex of all 29 tested embryos of six species of Darwins finches was determined successfully by PCR, using both primer sets. Hatchlings/nestlings and fledglings are also impossible to distinguish visually. This includes juveniles of sexually dimorphic species which are yet to moult in adult-like plumage and beak colouration. Furthermore, four species of Darwins finches are monomorphic, males and females looking alike. Therefore, sex assessment in the field can be a source of error, especially with respect to juveniles and mature monomorphic birds outside of the mating season. We caught 567 juveniles and adults belonging to six species of Darwins finches and only 44% had unambiguous sex-specific morphology. We sexed 363 birds by PCR, including individuals sexed based on marginal sex specific morphological traits (N=278) and birds which were impossible to classify in the field (N=39). For birds with marginal sex specific traits, PCR results revealed a 13% sexing error rate. This demonstrates that PCR based sexing can improve field studies on Darwins finches, especially when individuals with unclear sex-related morphology are involved. The protocols used here provide an easy and reliable way to sex Darwins finches throughout ontogeny, from embryos to adults.

molecular biology↗

bioRxiv2020

Phylogenetic and ontogenetic changes of the anatomical organization and modularity in the skull of archosaurs.

Comparative anatomy studies of the skull of archosaurs provide insights on the mechanisms of evolution for the morphologically and functionally diverse species of crocodiles and birds. One of the key attributes of skull evolution is the anatomical changes associated with the physical arrangement of cranial bones. Here, we compare the changes in anatomical organization and modularity of the skull of extinct and extant archosaurs using an Anatomical Network Analysis approach. We show that the number of bones, their topological arrangement, and modular organization can discriminate birds from non-avian dinosaurs, and crurotarsans. We could also discriminate extant taxa from extinct species when adult birds were included. By comparing within the same framework, juveniles and adults for crown birds and alligator (Alligator mississippiensis), we find that adult and juvenile alligator skulls are topologically similar, whereas juvenile bird skulls have a morphological complexity and anisomerism more similar to those of non-avian dinosaurs and crurotarsans than of their own adult forms. Clade-specific ontogenetic differences in skull organization, such as extensive postnatal fusion of cranial bones in crown birds, can explain this pattern. The fact that juvenile and adult skulls in birds do share a similar anatomical integration suggests the presence of a specific constraint to their ontogenetic growth.

evolutionary biology↗