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Abstract Numerous novel adaptations characterise the radiation of notothenioids, the dominant fish group in the freezing seas of the Southern Ocean. To improve understanding of the evolution of this iconic fish group, here we generate and analyse new genome assemblies for 24 species covering all major subgroups of the radiation, including five long-read assemblies. We present a new estimate for the onset of the radiation at 10.7 million years ago, based on a time-calibrated phylogeny derived from genome-wide sequence data. We identify a two-fold variation in genome size, driven by expansion of multiple transposable element families, and use the long-read data to reconstruct two evolutionarily important, highly repetitive gene family loci. First, we present the most complete reconstruction to date of the antifreeze glycoprotein gene family, whose emergence enabled survival in sub-zero temperatures, showing the expansion of the antifreeze gene locus from the ancestral to the derived state. Second, we trace the loss of haemoglobin genes in icefishes, the only vertebrates lacking functional haemoglobins, through complete reconstruction of the two haemoglobin gene clusters across notothenioid families. Both the haemoglobin and antifreeze genomic loci are characterised by multiple transposon expansions that may have driven the evolutionary history of these genes.
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Pathways to polar adaptation in fishes revealed by long‐read sequencing
Abstract Long‐read sequencing is driving a new reality for genome science in which highly contiguous assemblies can be produced efficiently with modest resources. Genome assemblies from long‐read sequences are particularly exciting for understanding the evolution of complex genomic regions that are often difficult to assemble. In this study, we utilized long‐read sequencing data to generate a high‐quality genome assembly for an Antarctic eelpout,Ophthalmolycus amberensis, the first for the globally distributed family Zoarcidae. We used this assembly to understand howO. amberensishas adapted to the harsh Southern Ocean and compared it to another group of Antarctic fishes: the notothenioids. We showed that selection has largely acted on different targets in eelpouts relative to notothenioids. However, we did find some overlap; in both groups, genes involved in membrane structure, thermal tolerance and vision have evidence of positive selection. We found evidence for historical shifts of transposable element activity inO. amberensisand other polar fishes, perhaps reflecting a response to environmental change. We were specifically interested in the evolution of two complex genomic loci known to underlie key adaptations to polar seas: haemoglobin and antifreeze proteins (AFPs). We observed unique evolution of the haemoglobin MN cluster in eelpouts and related fishes in the suborder Zoarcoidei relative to other Perciformes. For AFPs, we identified the first species in the suborder with no evidence ofafpIIIsequences (Cebidichthys violaceus) in the genomic region where they are found in all other Zoarcoidei, potentially reflecting a lineage‐specific loss of this cluster. Beyond polar fishes, our results highlight the power of long‐read sequencing to understand genome evolution.
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- PAR ID:
- 10401176
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Molecular Ecology
- Volume:
- 32
- Issue:
- 6
- ISSN:
- 0962-1083
- Page Range / eLocation ID:
- p. 1381-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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