Dorothea Heimeier

Escalating concern regarding the impacts of reduced genetic diversity on the conservation of endangered species has spurred efforts to obtain chromosome‐level genomes through consortia such as the Vertebrate Genomes Project. However, assembling reference genomes for many threatened species remains challenging due to difficulties obtaining optimal input samples (e.g., fresh tissue, cell lines) that can characterise long‐term conservation collections. Here, we present a pipeline that leverages genome synteny to construct high‐quality genomes for species of conservation concern despite less‐than‐optimal samples and/or sequencing data, demonstrating its use on Hector's and Māui dolphins. These endemic New Zealand dolphins are threatened by human activities due to their coastal habitat and small population sizes. Hector's dolphins are classified as endangered by the IUCN, while the Māui dolphin is among the most critically endangered marine mammals. To assemble reference genomes for these dolphins, we created a pipeline combining de novo assembly tools with reference‐guided techniques, utilising chromosome‐level genomes of closely related species. The pipeline assembled highly contiguous chromosome‐level genomes (scaffold N50: 110 MB, scaffold L50: 9, miniBUSCO completeness scores > 96.35%), despite non‐optimal input tissue samples. We demonstrate that these genomes can provide insights relevant for conservation, including historical demography revealing long‐term small population sizes, with subspecies divergence occurring ~20 kya, potentially linked to the Last Glacial Maximum. Māui dolphin heterozygosity was 40% lower than Hector's and comparable to other cetacean species noted for reduced genetic diversity. Through these exemplar genomes, we demonstrate that our pipeline can provide high‐quality genomic resources to facilitate ongoing conservation genomics research.

In temperate broadcast spawners, gamete release within the reproductive period is often triggered by exogenous cues such as phytoplankton concentration, seawater temperature, or day length (photoperiod). In Antarctica, little is known about cues to spawning, even within well-studied species, such as the sea urchin Sterechinus neumayeri. Here, we determine the spawning time in S. neumayeri using the time of appearance of embryos and larvae in plankton samples collected from the sea ice with a collapsible plankton net. Collections in McMurdo Sound and at three locations along the Victoria Land Coast of the Ross Sea [Cape Hallett (72°S), Terra Nova Bay (74°S), and Granite Harbour (77°S)] over a period of eight years showed that although there were differences in embryo and larval abundance between locations, multiple spawning events occurred during late October/early November to December, regardless of latitude, and appeared unrelated to moon phase. Exogenous spawning cues are limited in the Ross Sea during the early summer: sites are covered with sea ice with limited light penetration and water temperature is generally stable. As the spawning season on the Antarctic Peninsula and maritime Antarctic is also in October–December, we hypothesize that an endogenous cue plays a role in setting the time of gametogenesis and spawning, perhaps set by phytodetrital inputs in the late summer, as in the seasonally breeding sea urchins from the deep sea. Further research will be needed to test this hypothesis in Antarctic populations of S. neumayeri.

Escalating concern regarding the impacts of reduced genetic diversity on the conservation of endangered species has spurred efforts to obtain chromosome-level genomes through consortia such as the Vertebrate Genomes Project. However, assembling reference genomes for many threatened species remains challenging due to difficulties obtaining optimal input samples (e.g., fresh tissue, cell lines) that can characterize long-term conservation collections. Here, we present a pipeline that leverages genome synteny to construct high-quality genomes for species of conservation concern despite less-than-optimal samples and/or sequencing data, demonstrating its use on Hector’s and Māui dolphins. These endemic New Zealand dolphins are threatened by human activities due to their coastal habitat and small population sizes. Hector’s dolphins are classified as endangered by the IUCN, while the Māui dolphin is among the most critically endangered marine mammals. To assemble reference genomes for these dolphins, we created a pipeline combining de novo assembly tools with reference-guided techniques, utilizing chromosome-level genomes of closely related species. The pipeline assembled highly contiguous chromosome-level genomes (scaffold N50: 110 MB, scaffold L50: 9, miniBUSCO completeness scores >96.35%), despite non-optimal input tissue samples. We demonstrate that these genomes can provide insights relevant for conservation, including historical demography revealing long-term small population sizes, with subspecies divergence occurring ~20 kya, potentially linked to the Last Glacial Maximum. Māui dolphin heterozygosity was 40% lower than Hector’s and comparable to other cetacean species noted for reduced genetic diversity. Through these exemplar genomes, we demonstrate that our pipeline can provide high-quality genomic resources to facilitate ongoing conservation genomics research.

The major histocompatibility complex (MHC) is a highly polymorphic gene family that is crucial in immunity, and its diversity can be effectively used as a fitness marker for populations. Despite this, MHC remains poorly characterised in non‐model species (e.g., cetaceans: whales, dolphins and porpoises) as high gene copy number variation, especially in the fast‐evolving class I region, makes analyses of genomic sequences difficult. To date, only small sections of class I and IIa genes have been used to assess functional diversity in cetacean populations. Here, we undertook a systematic characterisation of the MHC class I and IIa regions in available cetacean genomes. We extracted full‐length gene sequences to design pan‐cetacean primers that amplified the complete exon 2 from MHC class I and IIa genes in one combined sequencing panel. We validated this panel in 19 cetacean species and described 354 alleles for both classes. Furthermore, we identified likely assembly artefacts for many MHC class I assemblies based on the presence of class I genes in the amplicon data compared to missing genes from genomes. Finally, we investigated MHC diversity using the panel in 25 humpback and 30 southern right whales, including four paternity trios for humpback whales. This revealed copy‐number variable class I haplotypes in humpback whales, which is likely a common phenomenon across cetaceans. These MHC alleles will form the basis for a cetacean branch of the Immuno‐Polymorphism Database (IPD‐MHC), a curated resource intended to aid in the systematic compilation of MHC alleles across several species, to support conservation initiatives.

Strong balancing selection on the Major Histocompatibility Complex (MHC) can lead to different patterns in gene frequencies and neutral genomic variation within species. We investigated diversity and geographic structure of MHC genes DQA and DQB, as well as their inferred functional haplotypes, from two regional populations (East and West Coast) of the endangered Hector’s dolphin (Cephalorhynchus hectori hectori) and the critically endangered Māui dolphin (Cephalorhynchus hectori maui) (West Coast, North Island), and contrasted these results with patterns from neutral microsatellites. The Māui had the lowest number of alleles for DQA (2) and DQB (3), consistent with strong genetic drift acting on this remnant population. However, the two retained DQA alleles are amongst the most divergent combinations of all four alleles found across the Hector’s metapopulation, potentially reflecting the retention of divergent alleles due to balancing selection. The high frequency of the divergent DQB*04 allele also gave this population the highest nucleotide diversity for DQB. Strong differentiation was evident for DQA, DQB, and DQA-DQB haplotypes between the regional populations of Hector’s dolphins (F_ST>0.213) and both subspecies (F_ST>0.311). Differentiation was generally greater than observed at neutral microsatellite loci, suggesting the influence of selection between geographically proximate East and West Coast populations. This might be the result of spatial differences in directional selection on those opposite coastlines. Additionally, measures of the ratio of non-synonymous to synonymous substitutions (d_N/d_S) were consistent with balancing selection over evolutionary time. Together, these results suggest a complex interplay of balancing selection, directional selection, local fidelity, and genetic drift.

We provide the first description and confirmation of expression for three Major Histocompatibility Complex (MHC) genes (DQA, DQB, and class I) in the New Zealand endemic Hector's dolphin (Cephalorhynchus hectori). Sequences were derived from cDNA generated from peripheral blood leukocyte mRNA collected during a temporary live capture of two Hector's dolphins. The complete exons 2 and 3 of DQA, DQB, and class I were amplified from cDNA to confirm their expression. The same primers proved reliable for amplification from genomic DNA. Locus identity was supported by phylogenetic relationships to MHC molecules from other closely related species. No evidence was found for duplication of DQ loci, but cloning of amplified exon 2 of the class I gene indicated duplication or triplication of this locus. However, the expression of multiple class I loci was uncertain as only one allele could be detected from clones of cDNA. Balancing selection was indicated by a significantly greater ratio of nonsynonymous to synonymous substitutions at all three loci and by the apparent transspecific sharing of alleles for DQA and DQB.