Delineation of species in the economically important agarophyte genera Gracilaria and Gracilariopsis has proven extremely difficult using available morphological characteristics. In this study, we examine the usefulness of two transcribed spacers for molecular systematic studies of these genera. The polymerase chain reaction was used to amplify the internal transcribed spacers (ITSs) and the intervening 5.8S ribosomal DNA of the nuclear ribosomal repeat region. In addition, a plastid spacer region and flanking regions of coding genes were amplified from the RUBISCO operon. Both regions were sequenced for individuals and populations of Gracilariopsis lemaneiformis (Bory) Dawson, Acleto, et Foldvik to determine the usefulness of these spacers in delimiting populations. These studies reveal that there is as much variation among individuals of a population as there is between individuals of geographically separate populations. In addition, the ITS spacer regions were compared between different species of Gracilariopsis and Gracilaria. The nuclear ITS spacer region is conserved at a species level in both genera and provides phylogenetically informative characters that can be used to examine species interrelationships among relatively closely related taxa. However, because of the difficulties of aligning this entire region among species from the two genera, the ITS region is not useful for examining intergenera relationships. ITS interspecies sequence comparisons indicate that Gracilariopsis lemaneiformis from California is significantly different from G. lemaneiformis from China and that a species of Gracilariopsis from Peru is more closely related to G. lemaneiformis from North Carolina than it is to the other Gracilariopsis species examined. In addition, these studies indicate that Gracilaria chilensis Bird, McLachlan, et Oliveira from New Zealand and Gracilaria tenuistipitata Chang et Xia from southeast Asia are as closely related as are Gracilaria verrucosa (Hudson) Papenfuss, G. pacifica Abbott, and Gracilaria robusta Kylin. Phylogenetic analysis of aligned plastid spacer sequences from Gracilaria and Gracilariopsis taxa provide similar conclusions about species relationships.

Two species of the colonial green flagellate family Volvocaceae are worldwide in distribution yet exhibit contrasting species structure. Geographically disparate isolates of Gonium pectorale Mueller can interbreed while isolates of Pandorina morum Bory behave quite differently. More than 20 sexually isolated subpopulations occur within this species; these have been termed “syngens” (sensu Sonneborn). Because prezygotic barriers to mating cause intersyngen pairings to fail, breeding analyses cannot be used to estimate genetic relatedness among the syngens of P. morum. DNA comparisons provide an alternative method of assessing genetic relatedness. We compared the nucleotide sequence of the internal transcribed spacer (ITS) region of the nuclear ribosomal repeat among clones of P. morum and of G. pectorale. Members of syngens of P. morum with distribution restricted to one small geographical area show great similarity. Likewise, members of any syngen of worldwide distribution show near uniformity, even those from different continents. However, the ITS sequence of each syngen differs from that of other syngens. In contrast, G. pectorale, which has an ITS region that is remarkably uniform throughout the world, appears to consist of a single syngen within North America and Europe by mating tests. The molecular data are in complete conformity with previous syngen assignment. Because the latter is based on mating affinity, with two complementary mating types per syngen, the evolution of new mating type pairs appears to be the basis of microevolution in these algae. We infer that either P. morum is a more ancient species than G. pectorale or that P. morum has a less stable genome. In either case, the biogeographic distribution of certain syngens may reflect climatological changes of the past.

Red algal parasites are common and have a unique type of development in which parasite nuclei are transferred to host cells and “control” host cell development. Previous phylogenetic studies have concentrated on parasites closely related to their hosts, termed adelphoparasites. A second set of parasites, usually classified in a different family or tribe from their host, termed alloparasites, have not been studied phylogenetically. This study concentrates on the wholly parasitic family, the Choreocolacaceae (Gigartinales). Using small subunit rDNA sequence data, we found that all the parasites studied are within the same family as their host. Our data support the placement of Holmsella, species of which parasitize Gracilaria and Gracilariopsis, in the order Gracilariales and suggest that Holmsella is an old parasitic genus. Most other species of the Choreocolacaceae parasitize species of the Rhodomelaceae. The one exception is the hyperparasitism between Harveyella mirabilis (Reinsch) F. Schmitz et Reinke (Rhodomelaceae) and the parasite Gonimophyllum skottsbergii Setchell (Delesseriaceae). The parasites Bostrychiocolax australis Zuccarello et West and Dawsoniocolax bostrychiae (Joly et Yamaguishi‐Tomita) Joly et Yamaguishi‐Tomita are placed within the tribe Bostrychiae as are their hosts. Harveyella mirabilis has a single origin and has switched hosts several times during its passage between the Atlantic and Pacific Oceans. Evidence does not support the continued recognition of the family Choreocolacaceae. Our results also indicate that the distinction between adelphoparasites and alloparasites is unwarranted, with a continuum between newly evolved parasites closely related to their hosts and parasites less closely related to their hosts.

In the initial stages of cell–cell interactions (spore germination and host penetration), the adelphoparasites Gardneriella tuberifera Kyl. and Gracilariophila oryzoides Setch. & Wilson form infection rhizoids that fuse directly with underlying host epidermal or cortical cells. In so doing, parasite nuclei and other organelles enter the cytoplasm of the host. The resulting heterokaryon may fuse with adjacent host cells either directly, via secondary pit connections, or by the dissolution or dislodgment of pit plugs from existing pit connections. The cell fusion events result in a heterokaryotic syncytium in which parasite nuclei replicate. In Gardneriella, formation of the syncytium induces surrounding host tissues to divide to form a photosynthetic callus. The internalized syncytium forms conjunctor and rhizoidal cells that fuse with host callus, eventually transforming the host callus into cells containing parasite nuclei. Gracilariophila does not induce surrounding host tissue to divide. Rather, division of the initial heterokaryotic tissue gives rise to the colorless mantle that protrudes from the host and forms reproductive structures. The heterokaryotic tissue also fuses with underlying host cells, thereby spreading parasite nuclei throughout adjacent host cells. In both these adelphoparasites, transformation of host cells by parasite nuclear invasion results in plastid dedifferentiation, an increase in mitochondria, autolysis of organelles, and accumulation of large amounts of floridean starch. The development and physiology of these parasites is similar to normal post‐fertilization processes in the hosts that give rise to carposporophytes and suggests that these adelphoparasites may have originated from perturbations of developmental pathways involved in their host's post‐fertilization development.

The phylogeny of Ectocarpus and Kuckuckia strains representing widely separated populations ?om both hemispheres was infmed from sequence analysis of the internal transcribed spacers of the nuclear ribosomal DNA (ITS 1-5.8s-ITS 2) and Lhe spucer regton i n the plastid-encoded ribulos~bis-phosphate-carboxylase (RUBISCO) cistron (partial rbc L-spac:er+artial rbcS) . Both sequences resulted in matching phylogenies, with the RUBISCO spacer regzon being most i r~o~a t i v~ at the h e 1 of genera and .specie.s and the i n t m a l transm'bed spacer sequences at the level of species and populations. Three major clades were -formed f q strains previously described morphology and physiolob us Kuckuckia, E. fasciculatus, and E. siliculosus, con$min,g the validity of the.w taxa. Ectocarpus and Kuckuckia are regarded as sibling taxa with respect to the outgroup .species Feldmannia simplex, Hincksia mitchelliae, and Pilayella littoralis. The clade f m e d by .sexual E. siliculosus .strains and most asexual Ectocarpus strains was subdivided into several clades that are consistent with geographical races within E. siliculosus. The inferred phylogeny of Ectocarpus corresponds generally with resu1t.r from cro.ss-f)rrilization expm'ment.s, mmphology, and lipid analysis. A hypothesis on the origzn and dispersal of E. siliculosus suggests several natural dispersal events during periods of global cooling as well as recent and po.rsibly anthropogenic dispmsal event.$.