Discovery of Genetically Distinct, Sympatric Coral Lineages With Temporal but Not Gametic Reproductive Isolation

Area of Science:

• Marine Biology
• Genetics
• Conservation Science

Background:

• Coral species diversity is often underestimated due to reliance on macromorphology.
• Morphological similarities can mask underlying behavioral and genetic divergence in coral communities.
• Previous research indicated unusual, bimodal spawning in Diploria labyrinthiformis in Curaçao.

Purpose of the Study:

• To investigate the genetic and behavioral divergence of Diploria labyrinthiformis in Curaçao.
• To determine the reproductive isolation mechanisms between sympatric lineages of D. labyrinthiformis.
• To assess the broader occurrence of distinct spawning populations across the Wider Caribbean Region.

Main Methods:

• Genome-wide sequencing to identify genetic clusters.
• Analysis of spawning data collected over multiple years (2013-2021).
• Cross-fertilization experiments using spring and autumn gametes.
• Field observations across 19 additional localities in the Wider Caribbean.

Main Results:

• Two distinct, genetically divergent lineages of D. labyrinthiformis were identified, with 93% of colonies exhibiting exclusive spring or autumn spawning.
• Macromorphology and depth did not differentiate the lineages, but genome-wide sequencing revealed significant genetic clusters (FST = 0.098).
• Successful fertilization and normal larval development in cross-fertilization experiments ruled out gametic isolation as a divergence factor.

Conclusions:

• Seasonal reproductive isolation (allochrony) is the primary barrier to gene flow between sympatric D. labyrinthiformis lineages.
• Allochronic isolation contributes to the formation and maintenance of cryptic coral diversity.
• Urgent identification and conservation of this hidden diversity are crucial for critically endangered reef-building corals.