# Negative Selection in Oreochromis niloticus × O. aureus Hybrids Indicates Incompatible Oxidative Phosphorylation (OXPHOS) Proteins

## TL;DR

This paper explores why hybrid tilapia hybrids are all male and suggests incompatibility in energy-producing proteins may prevent these hybrids from thriving in the wild.

## Contribution

The study introduces a novel method using amino acid substitutions to identify cytonuclear incompatibility in hybrid tilapia.

## Key findings

- A panel of 13 species-specific probes was developed to detect hybrid tilapia.
- 39.5% of screened experiments showed hybrid origin with mtDNA-nDNA mismatches.
- Three proteins showed significant negative selection, indicating cytonuclear incompatibility.

## Abstract

Crossing Oreochromis niloticus (On) females with O. aureus (Oa) males results in all-male progeny that are essential for effective tilapia aquaculture. However, a reproductive barrier between these species prevents commercial-scale yield. To achieve all-male progeny, the currently used practice is crossing admixed stocks and feeding fry with synthetic androgens. Hybrid tilapias escaping to the wild might impact natural populations. Hybrids competing with wild populations undergo selection for different stressors, e.g., oxygen levels, salinity, and low-temperature tolerance. Forming mitochondrial oxidative phosphorylation (OXPHOS) complexes, mitochondrial (mtDNA) and nuclear DNA (nDNA)-encoded proteins control energy production. Crossbred tilapia have been recorded over 60 years, providing an excellent model for assessing incompatibility between OXPHOS proteins, which are critical for the adaptation of these hybrids. Here, by comparing nonconserved amino acid substitutions, across 116 OXPHOS proteins, between On and Oa, we developed a panel of 13 species-specific probes. Screening 162 SRA experiments, we noted that 39.5% had a hybrid origin with mtDNA-nDNA allele mismatches. Observing that the frequency of interspecific mtDNA-nDNA allele combinations was significantly (p < 10−4) lower than expected for three factors, UQCRC2, ATP5C1, and COX4B, we concluded that these findings likely indicated negative selection, cytonuclear incompatibility, and a reproductive barrier.