The Genome of the Blind Soil-Dwelling and Ancestrally Wingless Dipluran Campodea augens: A Key Reference Hexapod for Studying the Emergence of Insect Innovations

A reference genome for the Andean cavefish Trichomycterus rosablanca (Siluriformes, Trichomycteridae): Building genomic resources to study evolution in cave environments

Animals living in caves are of broad relevance to evolutionary biologists interested in understanding the mechanisms underpinning convergent evolution. In the Eastern Andes of Colombia, populations from at least two distinct clades of Trichomycterus catfishes (Siluriformes) independently colonized cave environments and converged in phenotype by losing their eyes and pigmentation. We are pursuing several research questions using genomics to understand the evolutionary forces and molecular mechanisms responsible for repeated morphological changes in this system. As a foundation for such studies, here we describe a diploid, chromosome-scale, long-read reference genome for Trichomycterus rosablanca, a blind, depigmented species endemic to the karstic system of the department of Santander. The nuclear genome comprises 1 Gb in 27 chromosomes, with a 40.0× HiFi long-read genome coverage having an N50 scaffold of 40.4 Mb and N50 contig of 13.1 Mb, with 96.9% (Eukaryota) and 95.4% (Actinopterygii) universal single-copy orthologs (BUSCO). This assembly provides the first reference genome for the speciose genus Trichomycterus, serving as a key resource for research on the genomics of phenotypic evolution.

High-quality genomes reveal significant genetic divergence and cryptic speciation in the model organism Folsomia candida (Collembola)

The collembolan Folsomia candida Willem, 1902, is widely distributed throughout the world and has been frequently used as a test organism in soil ecology and ecotoxicology studies. However, it is questioned as an ideal "standard" because of differences in reproductive modes and cryptic genetic diversity between strains from various geographical origins. In this study, we obtained two high-quality chromosome-level genomes of F. candida, for a parthenogenetic strain (named as FCDK, 219.08 Mb, 25,139 protein-coding genes) and a sexual strain (named as FCSH, 153.09 Mb, 21,609 protein-coding genes), reannotated the genome of the parthenogenetic strain reported by Faddeeva-Vakhrusheva et al. in 2017 (named as FCBL, 221.7 Mb, 25,980 protein-coding genes), and conducted comparative genomic analyses of three strains. High genome similarities between FCDK and FCBL on synteny, genome architecture, mitochondrial and nuclear gene sequences support they are conspecific. The seven chromosomes of FCDK are each 25-54% larger than the corresponding chromosomes of FCSH, showing obvious repetitive element expansions and large-scale inversions and translocations but no whole-genome duplication. The strain-specific genes, expanded gene families and genes in nonsyntenic chromosomal regions identified in FCDK are highly related to the broader environmental adaptation of parthenogenetic strains. In addition, FCDK has fewer strain-specific microRNAs than FCSH, and their mitochondrial and nuclear genes have diverged greatly. In conclusion, FCDK/FCBL and FCSH have accumulated independent genetic changes and evolved into distinct species since 10 Mya. Our work provides important genomic resources for studying the mechanisms of rapidly cryptic speciation and soil arthropod adaptation to soil ecosystems.

Diversity and evolution of the P450 family in arthropods

The P450 family (CYP genes) of arthropods encodes diverse enzymes involved in the metabolism of foreign compounds and in essential endocrine or ecophysiological functions. The P450 sequences (CYPome) from 40 arthropod species were manually curated, including 31 complete CYPomes, and a maximum likelihood phylogeny of nearly 3000 sequences is presented. Arthropod CYPomes are assembled from members of six CYP clans of variable size, the CYP2, CYP3, CYP4 and mitochondrial clans, as well as the CYP20 and CYP16 clans that are not found in Neoptera. CYPome sizes vary from two dozen genes in some parasitic species to over 200 in species as diverse as collembolans or ticks. CYPomes are comprised of few CYP families with many genes and many CYP families with few genes, and this distribution is the result of dynamic birth and death processes. Lineage-specific expansions or blooms are found throughout the phylogeny and often result in genomic clusters that appear to form a reservoir of catalytic diversity maintained as heritable units. Among the many P450s with physiological functions, six CYP families are involved in ecdysteroid metabolism. However, five so-called Halloween genes are not universally represented and do not constitute the unique pathway of ecdysteroid biosynthesis. The diversity of arthropod CYPomes has only partially been uncovered to date and many P450s with physiological functions regulating the synthesis and degradation of endogenous signal molecules (including ecdysteroids) and semiochemicals (including pheromones and defense chemicals) remain to be discovered. Sequence diversity of arthropod P450s is extreme, and P450 sequences lacking the universally conserved Cys ligand to the heme have evolved several times. A better understanding of P450 evolution is needed to discern the relative contributions of stochastic processes and adaptive processes in shaping the size and diversity of CYPomes.

Multi-faceted analysis provides little evidence for recurrent whole-genome duplications during hexapod evolution

BACKGROUND

Gene duplication events play an important role in the evolution and adaptation of organisms. Duplicated genes can arise through different mechanisms, including whole-genome duplications (WGDs). Recently, WGD was suggested to be an important driver of evolution, also in hexapod animals.

RESULTS

Here, we analyzed 20 high-quality hexapod genomes using whole-paranome distributions of estimated synonymous distances (KS), patterns of within-genome co-linearity, and phylogenomic gene tree-species tree reconciliation methods. We observe an abundance of gene duplicates in the majority of these hexapod genomes, yet we find little evidence for WGD. The majority of gene duplicates seem to have originated through small-scale gene duplication processes. We did detect segmental duplications in six genomes, but these lacked the within-genome co-linearity signature typically associated with WGD, and the age of these duplications did not coincide with particular peaks in KS distributions. Furthermore, statistical gene tree-species tree reconciliation failed to support all but one of the previously hypothesized WGDs.

CONCLUSIONS

Our analyses therefore provide very limited evidence for WGD having played a significant role in the evolution of hexapods and suggest that alternative mechanisms drive gene duplication events in this group of animals. For instance, we propose that, along with small-scale gene duplication events, episodes of increased transposable element activity could have been an important source for gene duplicates in hexapods.