The chromosome-level genome of an invasive Australian population of the eastern mosquitofish (Gambusia holbrooki)
This study presents a high-quality, fully phased chromosome-level genome assembly of the invasive eastern mosquitofish (*Gambusia holbrooki*) from an Australian population, generated using PacBio HiFi and Hi-C sequencing to provide a critical resource for investigating its invasion biology, local adaptation, and evolutionary genomics.
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 This is an AI-generated explanation of a preprint that has not been peer-reviewed.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 world of biology as a massive, chaotic library where every living creature has its own instruction manual written in a language called DNA. For a long time, scientists could only read these manuals in blurry, fragmented snippets, like trying to understand a novel by looking at a few scattered words on a page. But recently, technology has given us a super-powered scanner that can read these manuals with incredible clarity, allowing us to assemble the whole story, chapter by chapter, and even see the differences between two copies of the same book. This is the world of genomics, where researchers map out the complete genetic code of organisms. Why does this matter? Because these instruction manuals hold the secrets to how animals survive, adapt to new environments, and sometimes, how they become so successful at taking over new places that they disrupt local ecosystems. Understanding these codes helps us figure out the rules of life and how species change over time.
Now, let's zoom in on a tiny, feisty fish called the Eastern mosquitofish (Gambusia holbrooki). Think of this little fish as a biological "super-tenant." Originally from the southeastern United States, it was moved around the world to help eat mosquito larvae, but it ended up becoming one of the most successful invaders on the planet, setting up shop in freshwater ponds everywhere from Europe to Australia. These fish are tough; they grow up fast, have huge families, and can handle a wide range of temperatures and salty water. In Australia, they've been living there for decades, adapting to the local heat and conditions. But until now, scientists didn't have a perfect, high-definition map of their genetic instruction manual to see exactly how they are so good at surviving.
This paper is the story of scientists finally building that perfect map. They took a single male mosquitofish from Townsville, Australia, and used a combination of advanced scanning technologies—specifically PacBio HiFi long-reads and Hi-C chromatin conformation sequencing—to piece together its entire genome. Think of PacBio HiFi as a high-resolution camera that takes clear, long photos of the DNA, and Hi-C as a way to figure out which photos belong on the same page of the book. The result is a fully "phased" chromosome-level assembly. In plain English, this means they didn't just get one blurry mix of the fish's two sets of chromosomes (one from mom, one from dad); they separated them into two distinct, complete versions, called HAP1 and HAP2.
The team found that the fish's genome is split into 24 chromosomes, with each of the two haplotypes (HAP1 and HAP2) being about 676 million letters (or base pairs) long. The quality of this map is stunningly high. The "contig N50" (a measure of how long the unbroken pieces of the puzzle are) is over 26 million letters, and the "scaffold N50" (how long the assembled pages are) is over 29 million letters. To put that in perspective, most of the chromosomes are assembled as single, continuous pieces without gaps. The scientists checked their work using a tool called BUSCO, which looks for a standard set of essential genes found in fish, and found that over 98.4% of these genes were present and complete in both versions of the map. They also checked the accuracy with a tool called Merqury, which gave the map a quality score of over Q66, meaning the text is incredibly accurate with very few typos.
One of the coolest parts of this discovery is that they found the "bookends" of the chromosomes. Just like a book has covers, chromosomes have special repeating sequences called telomeres at their tips. The scientists found these telomere signals at both ends of 13 chromosomes in the HAP1 version and 10 in the HAP2 version, with many others showing signals at least at one end. This suggests that for most of the fish's chromosomes, they have successfully assembled the entire length from one end to the other, a rare and valuable achievement.
The paper also compared this new Australian map to other existing maps of mosquitofish from Italy and a related species, the Western mosquitofish. They found that the overall structure of the chromosomes is very similar across these different fish, but they did spot some interesting differences. For instance, there appear to be specific inversions (where a section of the chromosome is flipped upside down) on chromosomes 23 and 24. The authors suggest these might be real, natural features of the fish's genome rather than mistakes in the map, because they saw similar flips when comparing the two different versions of the Australian fish's own DNA.
In short, this paper provides a crystal-clear, high-definition reference guide for the Eastern mosquitofish. It's not just a list of genes; it's a fully assembled, two-copy map that shows exactly how the DNA is organized. This resource is a game-changer for scientists who want to study how this invasive fish adapts to different environments, how it chooses its mates, and how it evolves so quickly. By having this perfect map, researchers can now start asking deeper questions about the genetic secrets behind the mosquitofish's incredible success as a global traveler.
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