At first glance, the spiny starfish populations inhabiting the Mediterranean and those living in the cold waters of the British Isles appear to belong to the same species with no major differences. However, their genome tells a different story: a scientific team has discovered that they exhibit an unusually high number of chromosomal inversions , large fragments of chromosomes that appear "upside down" in some individuals.
These investments affect 18,6% of the spiny starfish (Marthasterias glacialis) genome and appear to be associated with different environmental conditions . The finding suggests that genome organization may play a particularly important role in a marine species' ability to adapt to diverse environments.
Environment-associated chromosomal inversions
The research, led by the Doñana Biological Station (EBD-CSIC) and the University of Barcelona, ​​analyzed nearly 300 specimens from various regions of the Northeast Atlantic and the Western Mediterranean. "The most interesting finding is that the variants of these inversions are not randomly distributed among the studied populations, but rather appear associated with different geographical regions," explains Carlos Leiva, a researcher at the Doñana Biological Station. "Many of them are more frequent in warm, saline regions like the Mediterranean, while others appear associated with colder areas, such as the British Isles." Within these inverted regions, genes related to the response to thermal stress, osmoregulation, the immune response, and environmental perception were found. However, the researchers caution that this relationship will need to be validated through experimental studies. The results suggest, in any case, that the way the genome is organized may be especially important for understanding how a species adapts to different environments, in addition to point mutations in DNA. This mechanism could be especially relevant in the spiny starfish, a species whose populations maintain a high level of connection with each other despite living in environments with very different characteristics.
A mechanism for preserving local adaptations
The finding is particularly striking because of a phenomenon common in many marine organisms: the long-distance mobility of their larvae . Although adult starfish have more limited mobility, their larvae can travel hundreds of kilometers propelled by ocean currents. This movement facilitates genetic exchange and continuously mixes populations. In theory, this high degree of interconnectedness should hinder the emergence of significant adaptive differences between regions. However, the study demonstrates that there are areas of the genome capable of escaping this genetic mixing. Certain regions recombine, or "mix," much less than the rest of the genome, allowing them to remain distinct between populations. "These regions act as building blocks that are inherited together, thus preserving combinations of genes beneficial for living in specific environmental conditions, with differences in temperature or salinity," explains Marta MartÃn Huete, a predoctoral researcher at the University of Barcelona. In this way, chromosomal inversions could function as a kind of mechanism that allows certain genetic combinations favorable to a specific environment to be kept together, even when there is considerable genetic exchange between populations.
Nearly 300 starfish studied in the Atlantic and the Mediterranean
To determine if hidden differences existed in the species' genome, the scientific team analyzed samples from nearly 300 specimens of Marthasterias glacialis from 19 locations across the northeastern Atlantic and the western Mediterranean. The samples came from regions including the British Isles, the Azores, the European Atlantic coast, and the Mediterranean, allowing for comparisons of populations subjected to very different environmental conditions. The team analyzed thousands of markers distributed throughout the genome and compared them to a reference genome for the species. This approach allowed them to identify regions consistent with chromosomal inversions and study how these inversions were distributed among the different populations. Furthermore, the genetic data were combined with environmental information on temperature and salinity, enabling researchers to explore whether the chromosomal inversions might be related to local adaptation processes. The result was particularly striking: 18,6% of the genome exhibits this type of structural variation, a high proportion that points to the importance of chromosomal inversions in differentiating the populations analyzed.
Implications for the conservation of marine biodiversity
The study also reveals that a widely distributed species with connected and seemingly homogeneous populations can harbor significant adaptive diversity within its genome . This discovery has implications for the conservation of marine biodiversity, as different populations of the same species could respond differently to phenomena such as ocean warming or changes in salinity. Understanding this genetic diversity is especially relevant in the context of climate change, because the ability of species to respond to changes in temperature and other environmental conditions can determine their survival and future distribution. Furthermore, the results provide a new perspective on the mechanisms driving the evolution of marine biodiversity. The study demonstrates that changes in genome structure can be as important as specific differences in DNA in promoting species adaptation.
The spiny starfish as a model in the face of climate change
The next step will be to verify whether the identified chromosomal inversions actually have consequences for the biological characteristics of the individuals. "These results open the door to new studies to verify whether these chromosomal inversions actually translate into biological differences between individuals, such as greater heat tolerance, resistance to changes in salinity, variations in growth, or reproduction," explains Carlos Leiva. If future research confirms that these chromosomal inversions determine, at least in part, the spiny starfish's ability to adapt to heat or salinity, Marthasterias glacialis could become a model for studying how marine species can respond to an ocean marked by climate change. What began as a study of a starfish has thus revealed a new way to understand how marine organisms evolve and how they can maintain local adaptations even when their populations are connected by the movement of their larvae.
The discovery by the Spanish-French team, which also included the University of Guam, the Botanical Institute of Barcelona, ​​and the University of Rennes, underscores that genome organization is a key factor in evolution . The spiny starfish, with its 18,6% inversions, becomes a prime example of how chromosomal structure can conceal surprising adaptive diversity, offering valuable clues for anticipating how marine ecosystems will respond to future challenges.