Research on the “pregnant fish” continues

Egg-carrying females of Oryzias eversi and Adrianichthys oophorus. © Jana Flury


Some ricefish species from Sulawesi (Indonesia) practice what is known as “pelvic brooding”: they carry their eggs on their bodies until they hatch, rather than depositing them on plants like other ricefish species. Our researchers are delving deeper into this phenomenon. A new study on this topic examines the gene flow between ricefish species and attempts to identify the genes responsible for pelvic brooding.

The unique pelvic brooding ricefish species of the genera Oryzias and Adrianichthys, can only be found on the Indonesian island of Sulawesi. They carry their eggs attached to their belly until the embryos hatch, a reproductive strategy that was described as pelvic brooding, which is not known from the numerous other ricefish species in Southeast Asia. Previously, our researchers had already discovered the factors that favor pelvic brooding  and the role of so-called “innovative inflammations” in this process.

The latest study published in “Genome Biology and Evolution” examines the two lineages of pelvic-brooding ricefishes more closely to determine whether there was gene flow between the two species or if they independently evolved this trait. Researchers believe that these two lineages separated approximately 16 million years ago and that these younger pelvic brooding species diverged from each other about one to two million years ago.

Dr. Julia Schwarzer, senior author of the study and the Section Head of Evolutionary Genomics at the Museum Koenig in Bonn, summarizes the results of the latest study on the topic as follows: “We could not find any evidence of gene flow between the two pelvic brooding ricefish lineages. This is particularly intriguing for us, as it supports the hypothesis that this reproductive strategy likely evolved independently in both lineages.”

For example, it was very enlightening that the Adrianichthys from Lake Poso (on Sulawesi) share their habitat with egg-depositing ricefishes: “We were able to demonstrate gene flow between the egg-depositing species from Lake Poso and the pelvic-brooding Oryzias species.” This once again demonstrates how complex evolution can be and how extensive research is required to better understand it.

For this reason, a group of researchers including Julia Schwarzer, continues to investigate and uncover new insights into the phenomenon of “pelvic brooding in ricefishes.” It is already known on which chromosomes the code is hidden that is responsible for the formation of morphological characteristics of belly brooding, but the genes remain unknown. Morphological traits include, for example, long pelvic fins or a characteristic indentation at the bottom of the body of female ricefish.

Now, our researchers are addressing the questions of what has changed genetically since pelvic brooding evolved and how all the factors from previous studies and findings are related. In addition, a study will follow in which the entire genome of a pelvic brooding species will be sequenced.



Poso-See, Sulawesi, Indonesia: habitat of pelvic brooding Adrianichthys- and non-pelvic brooding Oryzias ricefishes. © Jana Flury


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