Unusual position of the genus Spirotaenia (Zygnematophyceae) among streptophytes revealed by SSU rDNA and rbcL sequence comparisons

Sexual reproduction by conjugation characterizes the genus Spirotaenia, and based on this character alone it has long been considered a member of the class Zygnematophyceae (Streptophyta). The similarity in chloroplast morphology between the unicellular Spirotaenia (Mesotaeniaceae) and the filamentous Spirogyro (Zygnemataceae) has led to the assumption that the two genera are evolutionarily related, a view apparently supported by rbcL sequence comparisons (McCourt et al. 2000). We sequenced the nuclear-encoded SSU rDNA and the rbcL genes from three Spirotaenia species and analysed their molecular phylogeny together with other streptophyte green algae (including representatives of the class Zygnematophyceae) and embryophytes using maximum likelihood, distance, maximum parsimony and Bayesian inference methods. The significance of the results was tested by bootstrap analyses, deletion of long-branch taxa and user-defined tree topologies. The two genes congruently revealed an unexpected position of the genus Spirotaenia within the streptophyte algae outside the class Zygnematophyceae, but associated with Chlorokybus. In contrast to a previously published rbcL phylogeny of conjugating green algae, our results suggest that Spirotaenia is not specifically related to Spirogyra and may not belong to the class Zygnematophyceae. The absence of the well-known zygnematophycean 1506 group I intron in all SSU rDNA sequences of Spirotaenia supports this conclusion.