Overcoming barriers to reef restoration: field-based method for approximate genotyping of Acropora cervicornis

Reef restoration efforts aim to enhance resilience by safeguarding the genetic diversity of coral populations. This could be facilitated by genotyping methods that are relatively inexpensive, and field-based. A potential method for assessing coral genotypic diversity arises from self-recognition phenomena. Past studies have shown that contact between tissues from the same genet (isogeneic) will fuse whereas tissue from different genets (allogeneic) will result in rejection. However, the accuracy of this method has been questioned. Here, we revisit the grafting method as a tool to estimate genotypic diversity in a Caribbean coral restoration target, Acropora cervicornis. Ten ramets of unknown genetic relation were arranged in 82 grafting tests consisting of 5-fragment bundles that replicated all possible combinations between ramets. After 10 weeks, we found that outcomes of acceptance and rejection were highly consistent (96.7% across all combinations and replicates). The proposed existence of 4 genets across the 10 ramets based on response outcomes was confirmed by two SNP-based genotyping methods. Both genet pairing (isogeneic or allogeneic pairs) and genetic distance significantly affected the odds of acceptance or rejection responses. Moreover, a significant correlation was found between the degree of fusion between fragments and their genetic distance, supporting that the most related ramets are accepted more strongly compared to those that are more unrelated. This field-based contact method can be a powerful tool to estimate genotypic diversity in coral nurseries, facilitating the management of genetic diversity within the nursery and genotype-level tracking of key traits like disease and bleaching resistance.