The first Antechinus reference genome provides a resource for investigating the genetic basis of semelparity and age-related neuropathologies

Plethora of New Marsupial Genomes Informs Our Knowledge of Marsupial MHC Class II

The major histocompatibility complex (MHC) plays a vital role in the vertebrate immune system due to its role in infection, disease and autoimmunity, or recognition of "self". The marsupial MHC class II genes show divergence from eutherian MHC class II genes and are a unique taxon of therian mammals that give birth to altricial and immunologically naive young providing an opportune study system for investigating evolution of the immune system. Additionally, the MHC in marsupials has been implicated in disease associations, including susceptibility to Chlamydia pecorum infection in koalas. Due to the complexity of the gene family, automated annotation is not possible so here we manually annotate 384 class II MHC genes in 29 marsupial species. We find losses of key components of the marsupial MHC repertoire in the Dasyuromorphia order and the Pseudochiridae family. We perform PGLS analysis to show the gene losses we find are true gene losses and not artifacts of unresolved genome assembly. We investigate the associations between the number of loci and life history traits, including lifespan and reproductive output in lineages of marsupials and hypothesize that gene loss may be linked to the energetic cost and tradeoffs associated with pregnancy and reproduction. We found support for litter size being a significant predictor of the number of DBA and DBB loci, indicating a tradeoff between the energetic requirements of immunity and reproduction. Additionally, we highlight the increased susceptibility of Dasyuridae species to neoplasia and a potential link to MHC gene loss. Finally, these annotations provide a valuable resource to the immunogenetics research community to move forward and further investigate diversity in MHC genes in marsupials.

Genome of the endangered eastern quoll (Dasyurus viverrinus) reveals signatures of historical decline and pelage color evolution

The eastern quoll (Dasyurus viverrinus) is an endangered marsupial native to Australia. Since the extirpation of its mainland populations in the 20th century, wild eastern quolls have been restricted to two islands at the southern end of their historical range. Eastern quolls are the subject of captive breeding programs and attempts have been made to re-establish a population in mainland Australia. However, few resources currently exist to guide the genetic management of this species. Here, we generated a reference genome for the eastern quoll with gene annotations supported by multi-tissue transcriptomes. Our assembly is among the most complete marsupial genomes currently available. Using this assembly, we infer the species' demographic history, identifying potential evidence of a long-term decline beginning in the late Pleistocene. Finally, we identify a deletion at the ASIP locus that likely underpins pelage color differences between the eastern quoll and the closely related Tasmanian devil (Sarcophilus harrisii).

Best genome sequencing strategies for annotation of complex immune gene families in wildlife

BACKGROUND

The biodiversity crisis and increasing impact of wildlife disease on animal and human health provides impetus for studying immune genes in wildlife. Despite the recent boom in genomes for wildlife species, immune genes are poorly annotated in nonmodel species owing to their high level of polymorphism and complex genomic organisation. Our research over the past decade and a half on Tasmanian devils and koalas highlights the importance of genomics and accurate immune annotations to investigate disease in wildlife. Given this, we have increasingly been asked the minimum levels of genome quality required to effectively annotate immune genes in order to study immunogenetic diversity. Here we set out to answer this question by manually annotating immune genes in 5 marsupial genomes and 1 monotreme genome to determine the impact of sequencing data type, assembly quality, and automated annotation on accurate immune annotation.

RESULTS

Genome quality is directly linked to our ability to annotate complex immune gene families, with long reads and scaffolding technologies required to reassemble immune gene clusters and elucidate evolution, organisation, and true gene content of the immune repertoire. Draft-quality genomes generated from short reads with HiC or 10× Chromium linked reads were unable to achieve this. Despite mammalian BUSCOv5 scores of up to 94.1% amongst the 6 genomes, automated annotation pipelines incorrectly annotated up to 59% of manually annotated immune genes regardless of assembly quality or method of automated annotation.

CONCLUSIONS

Our results demonstrate that long reads and scaffolding technologies, alongside manual annotation, are required to accurately study the immune gene repertoire of wildlife species.

Molecular evolution of vision-related genes may contribute to marsupial photic niche adaptations

Vision plays an essential role in the life of many animals. While most mammals are night-active (nocturnal), many have adapted to novel light environments. This includes diurnal (day-active) and crepuscular (twilight-active) species. Here, we used integrative approaches to investigate the molecular evolution of 112 vision-related genes across 19 genomes representing most marsupial orders. We found that four genes (GUCA1B, GUCY2F, RGR, and SWS2) involved in retinal phototransduction likely became functionally redundant in the ancestor of marsupials, a group of largely obligate nocturnal mammals. We also show evidence of rapid evolution and positive selection of bright-light vision genes in the common ancestor of Macropus (kangaroos, wallaroos, and wallabies). Macropus-specific amino acid substitutions in opsin genes (LWS and SWS1), in particular, may be an adaptation for crepuscular vision in this genus via opsin spectral sensitivity tuning. Our study set the stage for functional genetics studies and provides a stepping stone to future research efforts that fully capture the visual repertoire of marsupials.

A Chromosome-Scale Hybrid Genome Assembly of the Extinct Tasmanian Tiger (Thylacinus cynocephalus)

The extinct Tasmanian tiger or thylacine (Thylacinus cynocephalus) was a large marsupial carnivore native to Australia. Once ranging across parts of the mainland, the species remained only on the island of Tasmania by the time of European colonization. It was driven to extinction in the early 20th century and is an emblem of native species loss in Australia. The thylacine was a striking example of convergent evolution with placental canids, with which it shared a similar skull morphology. Consequently, it has been the subject of extensive study. While the original thylacine assemblies published in 2018 enabled the first exploration of the species' genome biology, further progress is hindered by the lack of high-quality genomic resources. Here, we present a new chromosome-scale hybrid genome assembly for the thylacine, which compares favorably with many recent de novo marsupial genomes. In addition, we provide homology-based gene annotations, characterize the repeat content of the thylacine genome, and show that consistent with demographic decline, the species possessed a low rate of heterozygosity even compared to extant, threatened marsupials.

Threatened Species Initiative: Empowering conservation action using genomic resources

Globally, 15,521 animal species are listed as threatened by the International Union for the Conservation of Nature, and of these less than 3% have genomic resources that can inform conservation management. To combat this, global genome initiatives are developing genomic resources, yet production of a reference genome alone does not conserve a species. The reference genome allows us to develop a suite of tools to understand both genome-wide and functional diversity within and between species. Conservation practitioners can use these tools to inform their decision-making. But, at present there is an implementation gap between the release of genome information and the use of genomic data in applied conservation by conservation practitioners. In May 2020, we launched the Threatened Species Initiative and brought a consortium of genome biologists, population biologists, bioinformaticians, population geneticists, and ecologists together with conservation agencies across Australia, including government, zoos, and nongovernment organizations. Our objective is to create a foundation of genomic data to advance our understanding of key Australian threatened species, and ultimately empower conservation practitioners to access and apply genomic data to their decision-making processes through a web-based portal. Currently, we are developing genomic resources for 61 threatened species from a range of taxa, across Australia, with more than 130 collaborators from government, academia, and conservation organizations. Developed in direct consultation with government threatened-species managers and other conservation practitioners, herein we present our framework for meeting their needs and our systematic approach to integrating genomics into threatened species recovery.

A Chromosome-Level Genome of the Agile Gracile Mouse Opossum (Gracilinanus agilis)

There are more than 100 species of American didelphid marsupials (opossums and mouse opossums). Limited genomic resources for didelphids exists, with only two publicly available genome assemblies compared with dozens in the case of their Australasian counterparts. This discrepancy impedes evolutionary and ecological research. To address this gap, we assembled a high-quality chromosome-level genome of the agile gracile mouse opossum (Gracilinanus agilis) using a combination of stLFR sequencing, polishing with mate-pair data, and anchoring onto pseudochromosomes using Hi-C. This species employs a rare life-history strategy, semelparity, and all G. agilis males and most females die at the end of their first breeding season after succumbing to stress and exhaustion. The 3.7-Gb chromosome-level assembly, with 92.6% anchored onto pseudochromosomes, has a scaffold N50 of 683.5 Mb and a contig N50 of 56.9 kb. The genome assembly shows high completeness, with a mammalian BUSCO score of 88.1%. Around 49.7% of the genome contains repetitive elements. Gene annotation yielded 24,425 genes, of which 83.9% were functionally annotated. The G. agilis genome is an important resource for future studies of marsupial biology, evolution, and conservation.