🧬 biology

A highly contiguous reference genome for the Argentine hake (Merluccius hubbsi Marinni 1923, Gadiformes, Merluccidae)

This study presents the first high-quality, de novo reference genome for the economically and ecologically vital Argentine hake (*Merluccius hubbsi*), which not only elucidates its evolutionary history and demographic fluctuations but also confirms its current robust genetic diversity and lack of recent inbreeding despite historical climate changes and intense fishing pressure.

Original authors: Melisa E Magallanes Alba, Agustin Baricalla, Alejandro D’Anatro

Published 2026-07-29

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Melisa E Magallanes Alba, Agustin Baricalla, Alejandro D’Anatro

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine the human body as a massive library containing every instruction needed to build and run a person. In the world of biology, this library is called a genome, and the books inside are written in a code made of four letters: A, C, G, and T. For a long time, scientists could only read tiny, torn-out pages of this library for many animals, making it hard to understand the whole story. But recently, technology has advanced, allowing us to stitch these pages back together into complete, high-resolution books. This is what we call a reference genome.

Why does this matter? Think of a species like a complex video game. If you want to fix a bug, understand how a character moves, or predict how they will react to a new environment, you need the full source code. Without it, you are just guessing. For fish that humans eat, like the Argentine hake, having this full "source code" is crucial. It helps scientists figure out if the fish populations are healthy, how they survived past ice ages, and how to keep fishing sustainable so the game doesn't crash. This paper is about finally writing down that complete source code for a very important fish.

The Argentine Hake's Big Book

Meet the Argentine hake (Merluccius hubbsi). It's a superstar of the ocean in the Southwest Atlantic, a fish so valuable that countries like Uruguay and Argentina rely on it for their fishing industries. But for a long time, scientists were trying to read this fish's genetic story using a flashlight in a dark room. They had fragments of information, but no complete picture. That changed when a team of researchers decided to build the first high-quality, "contiguous" (meaning all the pieces are glued together neatly) reference genome for this species.

Think of the genome assembly like solving a massive, billion-piece puzzle. The researchers didn't just use one type of puzzle piece; they used a hybrid strategy. They combined PacBio long reads, which are like long, winding ribbons of DNA that can span across tricky, repetitive sections, with NovaSeq short reads, which are like tiny, ultra-precise stamps that fix any spelling mistakes. By mixing these two, they managed to assemble a genome that spans approximately 425.3 Mb (megabases). To put that in perspective, if the genome were a book, it would be a thick volume with 28,998 protein-coding genes—the actual instructions for building the fish's body parts and functions.

The Genetic Health Check

Once they had the book, the scientists started reading it to check the fish's health. They were worried that years of heavy fishing might have hurt the population, causing inbreeding (like cousins marrying cousins in a small village), which usually leads to weak genetic health.

They looked for something called Runs of Homozygosity (ROH). Imagine your DNA as a long string of colored beads. If you have long stretches where the beads are exactly the same color on both strands, that's a sign of inbreeding. The researchers found something amazing: the Argentine hake had almost no long stretches of identical beads. The inbreeding coefficient was less than 0.01%. This suggests that despite the intense fishing pressure, the hake population is still huge, mixed together like a giant, healthy pot of soup, and hasn't suffered from a recent genetic crash. It's a sign of a robust, panmictic population where everyone is mixing genes freely.

A Time Machine to the Ice Age

The team also used the genome as a time machine to see how the fish population changed over thousands of years. Using a method called PSMC, they traced the "effective population size" (a way to count how many fish were actually reproducing) back through history.

The story they found is dramatic. About 100,000 years ago, the population hit a massive peak, with around 125,000 effective individuals. This likely happened during a warm period when the ocean habitats were perfect. But then, as the world cooled down leading into the Last Glacial Maximum (the peak of the last Ice Age), the population started to shrink, dropping to about 80,000 individuals by 20,000 years ago. This shows that the fish survived the freezing of the oceans, but the cold did squeeze their numbers down. Interestingly, this pattern was different from their European cousins, who found safe warm spots to hide in. The Argentine hake had to ride out the storm on the Patagonian shelf.

The Family Tree and the Great Migration

The researchers also assembled the fish's mitochondrial genome (a tiny, separate circle of DNA passed down only from mothers) to build a family tree for the whole Merluccius genus. They discovered that the genus likely started in the North Atlantic about 34 million years ago (in the late Oligocene).

The most exciting part of the family tree is where the Argentine hake fits in. The study confirms that the Argentine hake is the "sister taxon" (the closest relative) to the Pacific hakes. This means that the ancestors of the Argentine hake were the bridge that allowed the family to cross from the Atlantic Ocean into the Pacific Ocean during the Miocene epoch (around 15 million years ago). It was a crucial stepping-stone in their evolutionary journey.

What's Next?

While this new genome is a massive leap forward, the authors are careful to note it's just the beginning. They used DNA from a single fish, so while it tells us about the deep history and general health, it can't yet map out the tiny differences between fish in different specific spots today. Also, because they used a mix of sequencing methods, some of the very repetitive parts of the genome might be slightly underestimated compared to newer, ultra-long-read technologies.

However, this "source code" is now available for everyone. It's a foundational tool that will help scientists manage the fishery better, understand how the hake adapts to climate change, and ensure that this valuable resource remains on our plates for generations to come. The Argentine hake has finally been given its own voice in the library of life.

Technical Summary: A Highly Contiguous Reference Genome for the Argentine Hake (Merluccius hubbsi)

Problem Statement
The Argentine hake (Merluccius hubbsi) is a cornerstone demersal species in the Southwest Atlantic marine ecosystem and a primary commercial fishery resource for Uruguay and Argentina. Despite its significant ecological and economic value, the species has been subjected to intense, sustained fishing pressure. Effective conservation and stock assessment have been hindered by a critical lack of high-resolution genomic resources. Previous studies relied on low-density molecular markers or highly fragmented genome assemblies, leaving gaps in understanding the species' genomic architecture, evolutionary history, and current genetic health. There is an urgent need to transition from basic genetic monitoring to comprehensive genomic management to address population dynamics and adaptive capacity.

Methodology
The study employed a hybrid sequencing strategy to generate the first high-quality, de novo reference genome for M. hubbsi.

Sequencing: High molecular weight DNA from a single wild individual was sequenced using PacBio Sequel long-reads (targeting 10–20 kb inserts) and Illumina NovaSeq short reads (151 bp paired-end), achieving approximately 112X coverage.
Assembly: A hybrid de novo draft assembly was constructed using MaSuRCA. To reduce haplotypic redundancy, the draft was processed with Redundans. Scaffolding was performed using RagTag, leveraging the chromosome-level assembly of the closely related European hake (M. merluccius) as a reference for ordering and orientation. The assembly was polished with POLCA using Illumina reads to correct base-level errors.
Annotation: Structural annotation utilized a hybrid consensus approach combining BRAKER3 (using protein homology from Gadus morhua and OrthoDB) and LiftOn (transferring annotations from G. morhua). Functional annotation was performed using eggNOG-mapper.
Population and Demographic Analysis:
- Heterozygosity and Inbreeding: Illumina reads were mapped back to the reference to call SNPs. Runs of Homozygosity (ROH) were identified to calculate the genomic inbreeding coefficient ( $F_{ROH}$ ).
- Historical Demography: The Pairwise Sequentially Markovian Coalescent (PSMC) model was applied to a diploid consensus sequence to reconstruct historical effective population size ( $N_e$ ) fluctuations, scaled using a generation time of 3.5 years and a mutation rate of $1.0 \times 10^{-8}$ .
Phylogenetics: A complete mitogenome was assembled using GetOrganelle and annotated via MitoFish. A time-calibrated Bayesian phylogeny was inferred using BEAST v2.7.8 with 13 concatenated protein-coding genes and three Gadiformes outgroups, calibrated using fossil records and biogeographic events. Ancestral area reconstruction was performed using a discrete phylogeographic approach.

Key Contributions and Results

Genome Assembly: The final assembly spans approximately 425.3 Mb with 8,622 scaffolds. It features an N50 of 19.19 Mb, making it the second most contiguous Merluccius assembly to date, surpassed only by the chromosome-level assembly of M. merluccius. The assembly is 63% complete based on BUSCO analysis (actinopterygii_odb12).
Repetitive Landscape: Approximately 31.51% of the genome consists of repetitive sequences, dominated by interspersed repeats (24.53%), particularly LINEs and DNA transposons. A significant portion (18.82%) of repeats remained unclassified, suggesting a lineage-specific reservoir of transposable elements not present in public databases.
Gene Annotation: The study identified 28,998 protein-coding genes. After filtering for redundancy, 27,276 non-redundant protein sequences were retained, with 79.1% successfully assigned functional annotations (GO terms, KEGG pathways).
Genomic Health: Analysis of Runs of Homozygosity (ROH) revealed an almost complete absence of recent inbreeding ( $F_{ROH} < 0.01\%$ ) and a total ROH length of only 3.7 kb. This suggests the population maintains a massive, panmictic genetic pool with high gene flow, despite decades of intense fishing pressure.
Historical Demography: PSMC modeling indicates that the effective population size ( $N_e$ ) was stable between 50,000 and 60,000 individuals from 1,000,000 to 300,000 years ago. A population expansion occurred, peaking at ~125,000 individuals around 100,000 years ago (likely corresponding to interglacial conditions), followed by a decline coinciding with the cooling periods leading to the Last Glacial Maximum (LGM).
Phylogeny and Biogeography: The time-calibrated phylogeny places the origin of the Merluccius genus in the late Oligocene (~~34 MYA). The split between Old World (Euro-African) and New World (American) clades occurred in the middle Miocene (~~15 MYA). The analysis confirms M. hubbsi (Southwest Atlantic) as the sister taxon to the Pacific hake radiation, supporting the hypothesis that the colonization of the Southwest Atlantic was a transitional step preceding the invasion of the Pacific Ocean.

Significance
The authors assert that this high-quality reference genome provides an invaluable resource for future studies on stock assessment, functional adaptation, and the sustainable management of marine fisheries in the Southwest Atlantic. The findings indicate that M. hubbsi has maintained robust genetic diversity and navigated severe Pleistocene climatic oscillations without suffering recent genomic erosion. However, the authors note that while the single-ind genome analysis offers deep historical insights, it cannot capture contemporary fine-scale spatial population structure. Consequently, the paper emphasizes that this resource serves as a foundational framework for future multi-individual, population-level genomic screenings required to elucidate modern stock dynamics and local adaptation in the face of climate change and fishing pressure. The study also highlights the necessity of generating de novo repeat libraries for non-model marine teleosts, as public databases lack adequate representation for their lineage-specific transposable elements.