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Loss of hard chorion: An evolutionary dead-end for parental egg-care fish

Parental egg-care in fish traps them in an evolutionary dead-end through the loss of the chorion-hardening system, according to scientists from the Institute of Science Tokyo. Fish have diverse egg-caring strategies that have independently emerged multiple times across lineages. Comparative whole genome analyses of 240 fish species revealed a strong correlation between loss of the chorion-hardening system and parental egg-care, revealing the mechanisms behind the evolutionary bias that restricts egg-caring fish from becoming non-egg-carers.

As the most diverse group of vertebrates, fish have evolved a remarkable range of reproductive strategies to thrive across varied ecological niches. These strategies—ranging from non-egg-caring to live-bearing, mouth-brooding, and brood pouch care—have independently emerged across multiple lineages. While evolutionary transitions from non-egg-care to egg-care are common, the reverse appears rare. Despite this clear asymmetry, the molecular mechanisms underlying this evolutionary bias remain poorly understood.

Previous studies have noted that the toughness of the chorion—a protective envelope that hardens around fish eggs after fertilization—varies with reproductive strategy. Species without egg-care behavior typically develop a thick, hardened chorion to protect embryos from environmental stressors. In contrast, egg-care species tend to have fragile chorions.

Leveraging the fact that the chorion-hardening system has been well-studied in fish, Assistant Professor Tatsuki Nagasawa from the Institute of Science Tokyo led a collaborative study with scientists from Sophia University, Japan, to identify the molecular mechanisms underlying the evolutionary bias in fish reproductive strategies. Their findings were in Molecular Ecology on June 2, 2025.

"We hypothesized that by elucidating the molecular evolutionary processes of genes contributing to fish chorion formation, particularly focusing on egg-care species, it would be possible to uncover the details of fragile chorion formation and its evolutionary trajectory in these species," says Nagasawa. The team focused on a well-characterized cluster of genes responsible for chorion-hardening and assessed their evolutionary patterns using whole genome sequences from 240 species of Acanthopterygii fish, spanning 25 orders.

Their comparative genomic analysis revealed that although the chorion-hardening system had evolved over a long period in Acanthopterygii fish, the genes responsible for this function were repeatedly and independently lost as parental egg-care evolved. In particular, the team found that the alveolin gene—known to catalyze chorion-hardening after fertilization—was consistently lost across all egg-care species, regardless of the type of care provided.

Notably, within the order Syngnathiformes, where male fish carry eggs in brood pouches, the degree of alveolin gene loss was found to be proportional to the level of protection provided by the pouch. Species with fully enclosed, sac-like pouches showed complete pseudogenization of alveolin.

A strong correlation between alveolin gene loss and transition towards egg-care is evident in the fish species, suggesting that this is a major factor contributing to the evolutionary dead-end in these species. "Given the irreversibility of the shift toward egg-care, environmental changes that hinder parental care could significantly reduce fitness. Therefore, distinguishing between egg-care and non-care strategies is an important indicator from a conservation perspective," Nagasawa states.

This study also lends a new perspective to non-invasively understanding the reproductive strategies of endangered species based on DNA from whole genomes.

Overall, their findings suggest how gene loss associated with egg protection can lead to irreversible evolutionary changes, potentially constraining behavioral flexibility. These findings offer new insight into the genomic trade-offs underlying reproductive diversity—and the long-term consequences of parental care.