Search

Host specificity of symbionts is considered an important factor associated with sympatric speciation. Here, we examine host specificity and the degree of host-associated speciation in the barnacle Savignium milleporum, an obligate symbiont of the hydrocoral Millepora (the "fire coral"). Little morphological variability was revealed between barnacles collected from two morphs of the hydrocoral Millepora dichotoma (encrusting or branching) or from its congener Millepora platyphylla, but a molecular analysis revealed an unexpected pattern of DNA sequence divergence. The sequences of the 12S mitochondrial rDNA were nearly identical within each of the three barnacle populations (average sequence divergence <1%), and the sequences obtained for barnacles collected from the two different morphs of M. dichotoma differed considerably (ca. 9% average sequence divergence). However, S. milleporum collected from M. platyphylla were nearly identical to the barnacles from the branching M. dichotoma (<0.5% average sequence divergence). The pattern of speciation demonstrated by Savignium barnacles indicates the gradual colonization of similar hosts (i.e., sequential evolution), rather then "casual" colonization, as indicated for other systems. If this is indeed so, then symbiont phylogeny should roughly correlate with host phylogeny. Additionally, the data support the "rendezvous host" hypothesis, which invokes the opportunity of both sexes to meet as a major component for which selection favors the costly habit of host specificity.

Allorecognition is ubiquitous, or nearly so, amongst colonial invertebrates. Despite the prominent role that such phenomena have played both in evolutionary theory and in on the origin of the vertebrate immune system, unambiguous data on the transmission genetics of fusibility (i.e., the ability of two individuals to fuse upon tissue contact) is lacking for any metazoan outside of the phylum Chordata. We have developed lines of the hydroid Hydractinia symbiolongicarpus (Phylum Cnidaria) inbred for fusibility and here report results of breeding experiments establishing that fusibility segregates as expected for a single locus with codominantly expressed alleles, with one shared allele producing a fusible phenotype. Surveys of fusibility in field populations and additional breeding experiments indicate the presence of an extensive allele series.