Conspecific Gamete Precedence in Speciation: History, Evidence, and a Research Roadmap

Henry Arenas-Castro , Jan Engelstädter, Rhonda Snook, Loren Henry Rieseberg, Daniel Ortiz-Barrientos

Barriers to hybridization can act between gamete release and fertilisation. When an egg donor has simultaneous access to conspecific and heterospecific pollen or sperm, competitive interactions can bias fertilisation towards conspecific gametes, producing conspecific gamete precedence (CGP). Although such biases were first reported more than 250 years ago, CGP remains poorly integrated into the study of speciation. Here, we trace the history of CGP research from Joseph Gottlieb Kölreuter’s crossing experiments in the 1760s and review its reported occurrence in plants and animals. Since the 1970s, when CGP was first studied in an explicit speciation framework, the apparent occurrence of CGP has been reported in 93 case studies. However, the strength of this evidence varies because observed paternity can also reflect differences in gamete quantity and viability, non-competitive fertilisation success, and offspring survival. We therefore propose a common experimental and statistical framework for distinguishing CGP from these alternative sources of bias. Finally, we identify opportunities to investigate its mechanisms, evolutionary dynamics, and contribution to reproductive isolation. Standardising how CGP is tested and reported will make studies more comparable and allow its role in speciation to be assessed with greater confidence.