2 MORPHOLOGICALLY SIMILAR BIOLOGICAL SPECIES - CHONDRUS-PINNULATUS AND CHONDRUS-ARMATUS (GIGARTINACEAE, RHODOPHYTA)

The nature of the relationship between Chondrus pinnulatus (Harvey) Ok amura f. pinnulatus and C. pinnulatus f. armatus (Harvey) Yamada et Mi kami (Gigartinaceae, Rhodophyta) was investigated by comparative analy sis of their biogeography, phenologies, life histories, gross and vege tative morphology, crossability, and upper thermal tolerance. The form a pinnulatus has a more northerly distribution in Japan and adjacent w aters, exhibiting adaptation to the cooler regions, whereas the forma armatus has a more southerly range. The latter may be the result of a higher thermal tolerance. Both formae have a Polysiphonia-type life hi story and are similar in their internal vegetative morphology. They ca n, however, be distinguished by gross morphology: forma pinnulatus has wide, flattened axes, compressed to flattened ultimate segments and p roliferations, while forma armatus has narrow, compressed to subterete axes and subterete to terete ultimate segments and proliferations. Th ese differences persist in laboratory culture. All intraformae crosses were positive, with carpospores from the cross developing into fertil e F-1 tetrasporophytes releasing tetraspores that developed into dioec ious F-1 gametophytes, the female gametophytes of which formed normal cystocarps. This suggests that members of populations of each forma fr eely interbreed. Among interformae crosses, only some offspring derive d from geographically distant strains bore normal cystocarps in the F- 1 female gametophytes. Other crosses showed that interbreeding between populations of these two formae was blocked by various isolating mech anisms: incompatibility, hybrid inviability, and hybrid sterility. Rep roductive isolation between f. pinnulatus and f. armatus is virtually complete in wild populations, because hybrid populations have not been found in the wild. In addition, these two entities can be considered biological species that are also referred to the taxonomic species, C. armatus and C. pinnulatus, because they do not overlap with regard to the morphology of the ultimate segments and proliferations. subtle (b ut significant) gross morphological differences, partial interfertilit y between the two species, and deleterious hybridization in the area i n which they occur sympatrically suggest that their evolutionary diver gence was relatively recent.