Carex angustisquama‘s extreme life style keeps it a distinct species

Carex angustisquama is a sedge, a grass-like plant that grows in volcanic fields. It is a distinct species of Carex, but why? Koki Nagasawa and colleagues examined the plant’s genes to see how gene flow operated with its near relatives. They found it wasn’t that C. angustisquama was inherently isolated from its sister species. Instead, it was the hostile environment it called home that formed a physical barrier to gene flow.

You have to have an eye for detail to work on Carex taxonomy. There are over 2000 species, and many of these may be hybrids. One reason for this might be their holocentric chromosomes. When cells are splitting, spindle microtubules connect to kinetochores in the chromosome to pull it into a daughter cell. In most cells, the kinetochores sit in the centromere, a central section of the chromosome. In a holocentric chromosome, these kinetochores are spread through the chromosome. A holocentric chromosome can still be brought into a daughter cell if something breaks the chromosome, thanks to these distributed kinetochores. These holocentric chromosomes allow Carex to form hybrids easily. Carex also adapts well to local habitats.

Carex angustisquama lives in an unusual habitat. It lives in solfatara fields. These get their names from the volcano, Solfatara, famous for its sulphur. For botanists, the critical element of a solfatara field is the fumaroles, volcanic vents that continuously spew sulphide gases into the environment. The sulphide acidifies the soil, and the acidified soils have increased concentrations of Al³⁺, which is toxic to plant life. C. angustisquama dominates the most toxic areas.

C. angustisquama forms hybrids with some other species of Carex. Nagasawa and colleagues wanted to know how the genomes of C. angustisquama and its sister species interacted. “Interspecific hybridization between C. angustisquama and its closely related species implies the presence of interspecific gene flow, which may contribute to the adaptation of C. angustisquama to solfatara fields via the introgression of novel gene sets from hybrids with transgressive phenotypes not present in either parent,” they write. “Thus, it is crucial to determine whether gene flow between C. angustisquama and related species occurs to understand the evolutionary history of the extremophyte in solfatara fields.”

The team first gathered nuclear microsatellite genotypic data to examine the genetic structure of C. angustisquama and three hybrids. “These data offered insights into contemporary gene flow between C. angustisquama and other parental species,” write the scientists. “We then conducted a population demographic analysis to infer the direction and extent of historical gene flow between hybridizing species. Finally, we inferred the historical gene flow pattern among multiple Carex species, which grow in various habitats, to investigate the factors determining the gene flow pattern among the ecologically distinct species.”

“In our Bayesian clustering analysis aimed at revealing the genetic composition of three hybrid species, most of the hybrid individuals were classified into the F₁ genotypic class and the remaining few individuals were identified to be in the first backcross generation. Similar results were obtained from the hybrid triangle plot based on S and H_I, revealing the dominance of early hybrid generations and the asymmetrical hybridization into C. angustisquama in all hybrid species. Therefore, all hybrid species failed to form non-F₁ or non-BC1_Cang hybrid derivatives, and thus interspecific gene flow via these hybrids was limited.”

This lack of gene flow is interesting for the evolutionary history of C. angustisquama. Typically, gene flow provides plants with more genes to work with. Having more genes usually give plants more options for tackling environments. The authors note they could find the hybrids at multiple sites, so there was no reason C. angustisquama couldn’t bring in new genes. However, it lives in an extreme environment, and it is adapted to live in very particular conditions. Bringing new genes in could break the balance in the genome. Nagasawa and colleagues add that they have found that C. angustisquama has some of the least genetic diversity found among wild plants.

This maladaptation theory is support by Nagasawa and colleagues’ results. They have found that there is no problem for Carex to form hybrids. However, the hybrids were all early generations. There appeared to be no long term viability for hybrids. Either they adopted C. angustisquama‘s genes, or their offspring died out. For this reason, the authors argue that microhabitat segregation is the main reason for segregating C. angustisquama from other species.

These findings are consistent with Carex species elsewhere. It appears that Carex is a determined sedge that is capable of optimising to take advantage of whatever a microhabitat can offer.