Steven Vollmer and Stephen Palumbi have now resolved this apparent paradox with regard to three Caribbean species. Using the most extensive genetic analysis done so far on corals, they have shown that instead of there being three co-existing species, there are in fact only two, which hybridize to form the third. Their methods should be applicable to resolve other coral species, and should help to inform conservation programmes.
The authors focused their attention on three morphologically distinct forms of Acropora corals: the branched 'staghorn' coral A. cervicornis; the flatter 'elkhorn' coral A. palmata; and the morphologically intermediate A. prolifera. Intronic DNA sequence from two nuclear loci showed that the first two species are genetically distinct, unlike individuals from A. prolifera, all of which were heterozygous for the two loci, indicating that they are first-generation (F1) hybrids. Mitochondrial data revealed that some genetic mixing does occur between the hybrid and its parents, but at a low level, thereby confirming that A. prolifera arises from recent hybridization. In general, F1 hybrids — mules, for example — do not have much evolutionary potential as they cannot reproduce sexually. This is essentially true of A. prolifera as well. However, as they reproduce asexually by cloning themselves, these hybrid corals are virtually immortal, and endow the genus with many forms — a diversity that depends on the hybridization events themselves, on rare backcrosses with the parents and, curiously, on which parent provided the egg.
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