Biology subjects

McCullough, J.

Publications and source records attributed to McCullough, J..

7 recordsLinked to original sources

bioRxiv2026

Drift drives phenotypic evolution in a rapid island radiation

Understanding the processes that generate phenotypic diversity is central to explaining how new species form1,2. Evolutionary theory predicts that rapid evolution of signaling traits, such as feather coloration, can promote speciation3,4 but empirical support is inconsistent5,6. Phenotypic divergence of such traits is expected during speciation4, but these microevolutionary dynamics are rarely examined at macroevolutionary scales or linked to underlying population demography. Here, we leverage complete taxon sampling across an iconic insular bird radiation that helped shape early theories of allopatric speciation. We integrate whole-genome data with a comprehensive, fine-scale dataset of whole-body plumage coloration to directly test whether signaling trait evolution covaries with lineage diversification and to disentangle the roles of selection and drift. We find that lineages with faster rates of color evolution diversify more rapidly. Strikingly, rates of color evolution accelerate as genomic diversity declines, providing direct evidence that genetic drift--rather than strong sexual or ecological selection--can drive rapid phenotypic change in small, isolated insular populations. Together, these results provide compelling evidence that neutral demographic processes can accelerate the evolution of sexual signals and play a central role in generating phenotypic diversity during island radiations.

zoology↗

bioRxiv2024

Phylogenomics of a genus of "Great Speciators" reveals rampant incomplete lineage sorting, gene flow, and mitochondrial capture in island systems

The flora and fauna of island systems, especially those in the Indo-Pacific, are renowned for their high diversification rates and outsized contribution to the development of evolutionary theories. The total diversity of geographic radiations of many Indo-Pacific fauna is often incompletely sampled in phylogenetic studies due to the difficulty in obtaining single island endemic forms across the Pacific and the relatively poor performance of degraded DNA when using museum specimens for inference of evolutionary relationships. New methods for production and analysis of genome-wide datasets sourced from degraded DNA are facilitating insights into the complex evolutionary histories of these influential island faunas. Here, we leverage whole genome resequencing (20X average coverage) and extensive sampling of all taxonomic diversity within Todiramphus kingfishers, a rapid radiation of largely island endemic Great Speciators. We find that whole genome datasets do not outright resolve the evolutionary relationships of this clade: four types of molecular markers (UCEs, BUSCOs, SNPs, and mtDNA) and tree building methods did not find a single well-supported and concordant species-level topology. We then uncover evidence of widespread incomplete lineage sorting and both ancient and contemporary gene flow and demonstrate how these factors contribute to conflicting evolutionary histories. Our complete taxonomic sampling allowed us to further identify a novel case of mitochondrial capture between two allopatric species, suggesting a potential historical (but since lost) hybrid zone as islands were successively colonized. Taken together, these results highlight how increased genomic and taxon sampling can reveal complex evolutionary patterns in rapid island radiations.

zoology↗

bioRxiv2022

Complex plumages spur rapid color diversification in island kingfishers (Aves: Alcedinidae)

Oceanic islands are cradles for diversity. Differences in predation pressures and lack of competition on islands are thought to drive both phenotypic and species diversification. While most work exploring these patterns has focused on life history, behavioral and morphological traits, many island species are uniquely colorful. Yet, a recent study of island bird coloration found that insular species are duller than continental species. Whether such shifts in color are associated with increased rates of color evolution on islands remains unknown. Here, we incorporate geometric morphometric techniques to study plumage color diversity in a speciose clade of colorful birds that inhabit nearly all areas of the globe--kingfishers (Aves: Alcedinidae). In particular, we test two hypotheses: (i) that plumage complexity enhances interspecific rates of color evolution and (ii) that plumage color diversity is elevated on islands. Our results show that more complex plumages result in more diverse colors among species and plumage color evolves faster on islands. Importantly, we found that insular species did not have more complex plumages than their continental relatives. Thus, complexity may be a key innovation that facilitates response to divergent (or relaxed) selection pressures on islands. Lack of support for competition driving rates of evolution along different color axes hints at an allopatric model of color evolution in which species adapt to local conditions on different islands. This work demonstrates how a truly multivariate treatment of color data can reveal evolutionary patterns that might otherwise go unnoticed.

evolutionary biology↗

bioRxiv2020

A Duplicated ESCRT-III Factor Blocks Enveloped Virus Budding Without Inhibiting Cellular ESCRT Functions

Many enveloped viruses require the endosomal sorting complexes required for transport (ESCRT) pathway to exit infected cells. This highly conserved pathway mediates essential cellular membrane fission events, which restricts the acquisition of adaptive mutations to counteract viral co-option. Here, we describe duplicated and truncated copies of the ESCRT-III factor CHMP3 that block ESCRT-dependent virus budding and that arose independently in New World monkeys and mice. When expressed in human cells, these retroCHMP3 proteins potently inhibit release of retroviruses, paramyxoviruses, and filoviruses. Remarkably, retroCHMP3 proteins have evolved to reduce interactions with other ESCRT-III factors and to have little effect on cellular ESCRT processes, revealing routes for decoupling cellular ESCRT functions from viral exploitation. The repurposing of duplicated ESCRT-III proteins thus provides a mechanism to generate broad-spectrum viral budding inhibitors without blocking highly conserved essential cellular ESCRT functions.

immunology↗

bioRxiv2019

Membrane constriction and thinning by sequential ESCRT-III polymerization

The Endosomal Sorting Complexes Required for Transport (ESCRTs) mediate diverse membrane remodeling events. These activities typically require ESCRT-III proteins to stabilize negatively-curved membranes, although recent work has indicated that certain ESCRT-IIIs also participate in positive-curvature membrane shaping reactions. ESCRT-IIIs polymerize into membrane-binding filaments, but the structural basis for negative versus positive membrane curvature shaping by these proteins remains poorly understood. To learn how ESCRT-IIIs shape membranes, we determined structures of human membrane-bound CHMP1B-only, membrane-bound CHMP1B+IST1, and IST1-only filaments by electron cryomicroscopy. Our structures show how CHMP1B first polymerizes into a single-stranded helical filament, shaping membranes into moderate-curvature tubules. Subsequently, IST1 assembles a second strand upon the CHMP1B filament, further constricting the membrane tube and reducing its diameter nearly to the fission point. Each step of constriction, moreover, thins the underlying bilayer and lowers the barrier to membrane fission. Together, our structures reveal how a two-component, sequential polymerization mechanism drives membrane tubulation, tube constriction, and bilayer thinning.

biophysics↗

bioRxiv2019

A tunable microfluidic device enables cargo encapsulation by cell- or organelle-sized lipid vesicles comprising asymmetric lipid monolayers

Cellular membranes play host to a wide variety of morphologically and chemically complex processes. Although model membranes, like liposomes, are already widely used to reconstitute and study these processes, better tools are needed for making model bilayers that faithfully mimic cellular membranes. Existing methods for fabricating cell-sized (m) or organelle-sized (tens to hundreds of nm) lipid vesicles have distinctly different requirements. Of particular note for biology, it remains challenging for any technique to efficiently encapsulate fragile cargo molecules or to generate liposomes with stable, asymmetric lipid-leaflets within the bilayer. Here we describe a tunable microfluidic device and protocol for fabricating liposomes with desired diameters ranging from ~10 m to ~100 nm. Lipid vesicle size is templated by the simple inclusion of a polycarbonate filter within the microfluidic system and tuned with flow rate. We show that the vesicles made with our device are stable, unilamellar, lipid asymmetric, and capable of supporting transmembrane protein assembly, peripheral membrane protein binding, as well as soluble cargo encapsulation (including designer nanocages for biotechnology applications). These fabricated vesicles provide a new platform for studying the biophysically rich processes found within lipid-lipid and lipid-protein systems typically associated with cellular membranes.

bioengineering↗

bioRxiv2018

The ESCRT-III proteins IST1 and CHMP1B assemble around nucleic acids

ESCRT-III proteins can promote inside-out or outside-in membrane tubulation and fission. In addition, several observations suggest that ESCRT factors may also associate with nucleic acids during development, different stages of the cell cycle, and during retro-transposition of parasitic nucleic acids like LINE1 elements. Two ESCRT-III subunits, IST1 (aka CHMP8) and CHMP1B, can coassemble as an external protein coat around liposomes in vitro and around recycling endosomal tubules in living cells. Here we show that recombinant IST1 and CHMP1B can also copolymerize into double stranded filaments that surround nucleic acids. Electron cryo-microscopy reconstructions of nucleic acid-bound IST1-CHMP1B copolymers revealed that the polynucleotides track along a binding groove formed between filaments of the inner CHMP1B strand. The well-ordered structures also reveal that the C-terminal tails of CHMP1B subunits extrude through the outer IST1 layer to the tube exterior. As a result, the MIT domain binding motifs of both CHMP1B and IST1 are arrayed on the outer surface of the copolymer, where they could bind and recruit MIT domain-containing co-factors, such as the SPASTIN ATPase or the USP8 ubiquitin protease. Our structure raises the possibility that ESCRT-III proteins may form nucleic acid complexes in mammalian cells.

biochemistry↗