The Green Hydra Symbiosis. VI. What is the Role of Maltose Transfer from Alga to Animal?

Maltose is the principal form in which organic carbon is transferred from symbiont to host in green hydra. Associations can be established between aposymbiotic `European' hydra and certain strains of Chlorella originally isolated from Paramecium bursaria which vary in the amount of maltose that they release in culture. These strains were characterized as either `high releasers' (40-170 ng of maltose released per 10⁶ cells per 10 min) or `low releasers' (0.3-2 ng of maltose per 10⁶ cells per 10 min). After establishment of stable associations which had remained constant through several host generations, high releasers were estimated to transfer at least ten times as much maltose to their hosts as low releasers. Both types of Chlorella established similar population sizes in hydra, suggesting that, although maltose release is known to be essential for successful infection, the amount released is of little importance in determining eventual symbiont population size. There were no marked and consistent differences in budding rate in the light between hydra infected with high or low releasers. Hydra infected with their own native symbionts sometimes grew more slowly in the light than those infected with foreign Chlorella, and, in the dark, the slower growth become more marked under conditions of starvation. These results were interpreted in terms of the possibility that native symbionts are more efficient at exploiting their habitats than foreign. Thus, the level of glycogen in host tissues in hydra grown in the dark was less if native symbionts were present than if low releasers were present.