Cichlids: Record-holder for diversity

Molecular and evolutionary biologist Dr Astrid Böhne, Head of the Comparative Genomics Section at the LIB, explains in this interview how modern genomic research is changing our understanding of sex, speciation and biological diversity.

What does ‘diversity’ mean in evolutionary biology, and how does this term differ from ‘biodiversity’?

Our research focuses primarily on the diversity of sex chromosomes. Unlike in mammals, where the XY system is very stable, we find many different mechanisms of sex determination in fish. In cichlids, a wide variety of chromosomes and gene loci can influence how an individual’s sex develops.

At the same time, we are interested in how this diversity in sex determination and reproduction relates to biodiversity. Cichlids are an exceptionally species-rich family of fish. We would like to understand whether the diversity of their sex systems may have contributed to the emergence of this enormous species diversity.

Why are African cichlids particularly well suited to research into diversity and evolution?

Cichlids have undergone explosive speciation. There are an incredible number of species, which vary greatly in size, colouration and lifestyle. At the same time, they are still relatively closely related to one another in evolutionary terms. This makes them ideal for comparative genomic studies.

We find the species of Lake Tanganyika in East Africa particularly fascinating, as the lake is home to an exceptionally high diversity of different cichlid lineages.

Cichlids are considered to hold the record for the diversity of sex chromosomes. What makes this diversity so extraordinary?

For a long time, we assumed that sex determination must be highly conserved in evolutionary terms. After all, it is a fundamental developmental decision for every organism.

Today, we can see that this is actually the exception rather than the rule. If we look beyond mammals to fish, amphibians and insects, we find an astonishing variety of mechanisms.

Nevertheless, certain genes keep cropping up. We sometimes refer to them as the ‘usual suspects’. However, we do not yet fully understand why the systems change so frequently.

Why do reproductive systems and sex chromosomes in cichlids evolve so rapidly, and what can we learn from this about the origin of new species?

One hypothesis is that differences in sex determination may contribute to populations drifting apart and eventually giving rise to new species. Whether the diversity of sex determination systems is in fact a driving force behind the extraordinary speciation seen in cichlids is one of the central questions of our research.

What role does comparative genomics play in highlighting such forms of diversity?

Comparative genomics is our most important tool. It shows just how dynamic genomes can be and helps us to understand how the sections of the genome involved in sex determination have evolved.

What significance do terms such as ‘male’, ‘female’ or ‘sex’ have for biological research?

For a long time, research was heavily influenced by a binary perspective. We now know that biological reality is often considerably more complex.

The more organisms we study, the clearer it becomes just how limited some of our previous categories were. That is why we are focusing intensively on how terms are defined and how language influences research.

How can research into diversity contribute to the protection of biodiversity?

We can only protect what we understand.

Genomic methods help us to reveal hidden diversity. Sometimes it turns out that a supposed species actually consists of several genetically distinct lineages. Such findings can have a direct impact on conservation measures.

Cichlids, in particular, often live in very specific habitats. Many species are found only in individual lakes. When these ecosystems are lost, entire species often disappear forever.

Is nature more diverse than the terms and models currently available can capture?

I am convinced of that. The more broadly we study biodiversity, the more we realise just how diverse nature really is, and how much our understanding has long been shaped by a small number of model organisms.

What advice would you give to young people who want to get into research today?

Stay open-minded and curious.

We are currently witnessing enormous technological progress. This enables us to answer questions about evolution that seemed out of reach just a few years ago.

There is still an incredible amount to discover. That’s what makes this research so exciting.

Further reading

How do researchers talk about sex, and how do scientific terms influence our view of biological diversity? These are among the questions addressed in a recent publication by the international Tree of Sex Consortium, to which Dr Astrid Böhne has contributed.

Publication:

Caitlin E McDonough-Goldstein, Soleil E Young, Maurine Neiman, Sadye Paez, Nicole Valenzuela, Cibele G Sotero-Caio, Daniel L Jeffries, Octavio Manuel Palacios-Gimenez, Jessica K Abbott, Chiara Benvenuto, Ann Kathrin Huylmans, Sara Calhim, J Antonio Baeza, Andrew J Mongue, Erica L Larson, Lucija Andjel, Rainer Melzer, Elizabeth Dietz, Aurora Ruiz-Herrera, Tree of Sex Consortium , Navigating the semantic labyrinth of “sex” in the study of reproductive trait evolution, Integrative and Comparative Biology, 2026;, icag093, https://doi.org/10.1093/icb/icag093