THACKER, R.W.*; STARNES, S.: Host Specificity of Symbiotic Cyanobacteria in Marine Sponges

Although marine sponges can host a variety of cyanobacterial and bacterial symbionts, it is often not known whether these symbionts are generalists that occur in a variety of host species or specialists that occur only in certain species or populations of sponges. We observed the filamentous cyanobacterium Oscillatoria spongeliae in collections of the sponge Dysidea cf. herbacea and similar cyanobacteria in collections of D. cf. granulosa and D. cf. avara. We hypothesized that host-specific strains of cyanobacteria are found in each sponge species. To test this hypothesis, we extracted total genomic DNA from specimens of all three Dysidea species collected from several locations on Guam, and including two color forms of D. cf. herbacea. We used sponge-specific PCR primers to amplify a 700 bp fragment of sponge nuclear ribosomal DNA, including the ITS-2 region, and we used cyanobacteria-specific PCR primers to amplify 1300 bp of cyanobacterial 16S ribosomal DNA. After sequencing these products, we constructed phylogenies for both the symbiotic cyanobacteria and the host sponges. These phylogenies show that a separate cyanobacterial strain is found in each sponge species. We found little sequence variation within sponge species or cyanobacterial strains, with no consistent differentiation of sponges or cyanobacteria among locations or among sponge color forms. Log-determinant distance matrices from each group were significantly positively correlated, indicating that more closely related sponges host more closely related cyanobacteria. These data suggest a high potential for coevolution, and possibly cospeciation, of symbiotic cyanobacteria and their host sponges.