The evolution of complex phenotypes like reproductive strategies is challenging to understand as they often depend on multiple adaptations, which only jointly result in a specific functionality. Sulawesi ricefishes (Adrianichthyidae) evolved a reproductive strategy termed pelvic brooding. In contrast to the more common transfer brooding, female pelvic brooders carry an egg-bundle connected to their body for weeks until the fry hatches. To examine the genetic architecture of pelvic brooding, we crossed the pelvic brooding Oryzias eversi and the transfer brooding O. nigrimas (species divergence time: ~ 3.6 my). We hypothesize, that a low number of loci and modularity have facilitated the rapid evolution of pelvic brooding. Traits associated to pelvic brooding, like rib length, pelvic fin length and morphology of the genital papilla were correlated in the parental species but correlations were reduced or lost in their F1 and F2 hybrids. Using the Castle-Wright estimator, we found that generally few loci underlie the studied traits. Further, both parental species showed modularity in their body plans. In conclusion, morphological traits related to pelvic brooding were based on a few loci and the mid-body region likely could evolve independently from the remaining body parts. Both factors presumably facilitated the rapid evolution of pelvic brooding.

We photographed preserved specimen and measured distances using ImageJ.

There is missing data in the O. eversi backcross hybrids. This is due the Covid-19 lockdown and the labs were closed.