Recent evolution of large offspring size and post-fertilization nutrient provisioning in swordtails

Organisms have evolved diverse reproductive strategies that impact the probability that their offspring survive to adulthood. Using morphological measurements in embryos and fry, gene expression analysis, and hybrid crosses, we describe divergence in reproductive strategy between two closely related species of swordtail fish (Xiphophorus), which have internal fertilization and give birth to free-swimming fry. We find that one species, X. malinche has evolved larger offspring than its closest relative X. birchmanni and dwarfs the offspring size of other species in the genus. The fry of X. malinche are more resilient to starvation than X. birchmanni, hinting that the evolution of large offspring size could be an adaptation to the challenging environments in which X. malinche are born. We also find evidence that X. malinche mothers provision nutrients to their offspring during embryonic development, the first time this process has been documented in the Xiphophorus genus. Moreover, in the ..., , # Recent evolution of large offspring size and post-fertilization nutrient provisioning in swordtails

### Payne et al 2025. Proc B.

This repository contains all code and accompanying input data files that generated the results
presented in this study. Please reference our paper for detailed methods. All code and materials
included in this repository can also be found on GitHub
at [github.com/cypayne/swordtail-offspring-size](https://github.com/cypayne/swordtail-offspring-size).

## Abstract

Organisms have evolved diverse reproductive strategies that impact the probability that their offspring survive to adulthood. Using morphological measurements in embryos and fry, gene expression analysis, and hybrid crosses, we describe divergence in reproductive strategy between two closely related species of swordtail fish (Xiphophorus), which have internal fertilization and give birth to free-swimming fry. We find that one species, X. malinche, which lives in high-elevation environments, has ev...,