Species delineation and autecology of Spirogyra

The filamentous green algae Spirogyra is found in a wide range of habitats, including small stagnant water bodies, ditches as well as the littorals of lakes and streams. Since the genus is easily recognized due to its spiral chloroplasts, the genus distribution is well recorded. This doesnt hold true for the species dispersion because of complications in species determination, for which sexual reproduction stages including ripe hypnozygotes are essential. One cardinal problem is that Spirogyra can be found mainly in its vegetative stage. As a consequence, we know a genus with an exceptionally high potential as indicator for the ecological status of limnic habitats, but hold hardly information about the requirements of single species. In this context, the induction of the mating process until the ripening of the hypnozygotes is one central objective of this project, since sexual reproduction stages and morphology of ripe hypnozygotes are diagnostic criteria for traditional species determinations. Special attention will be paid to the basically two-layered mesospore, which consists of sporopollenin. As already annotated in previous studies, inconsistent and vague illustrations of the spore walls hamper the species determination. In our proposed work, we will amend SEM pictures of prepared mesospores (Acetolysis) to illustrate the spore wall structures more clearly. A few studies indicate, that zygote morphology may not reflect true relationships and species delineations. There exist polyploid species complexes with identical DNA sequences showing different cell and zygote morphology. As a consequence, the traditional morphospecies-concept needs to be reviewed and adapted. This is one more target of the proposed project, where traditional characters will be compared with primary and secondary structures of the Internal Transcribed Spacer 2 regions of the rDNA, cellular DNA-contents and chromosome numbers. The proposed work is designed as a contribution on the autecology and species delineation of Spirogyra including the compilation of a database containing morphological characteristics and ITS sequences. Allocation of identified unialgal strains is one more target of our study, since Spirogyra cultures are available only in limited numbers. Our project will provide clonal cultures for further molecular and ecophysiological investigations.

This project dealt with the ecology and relationships of the filamentous alga Spirogyra. An intense field collection in Central Europe (Spirogyra was found in 133 out of 314 sites) revealed an ecological optimum of this genus in freshwaters of medium nutrient concentrations. In total, 333 different filament morphotypes were classified based on 18500 measurements from vegetative filaments, which were then clustered into 13 morphogroups. These groups seem to be applicable for bioindication purposes. Zygospores are essential for species identification of Spirogyra. We therefore tried to induce sexual reproduction. Altogether, 831 experiments were conducted, but they resulted in a success rate of only 5 %. We were not able to find a single trigger for sex induction. From this low success rate, we conclude that some strains might have lost the ability to reproduce sexually. This hypothesis is fortified by the findings of polyploidal stages and a very fast evolution rate within this genus. To quantify the cellular DNA content, we developed a method based on DAPI staining and microdensitometry. Maximum cDNA content was obtained from Spirogyra pseudomaxima with 2.46 pg and the minimum value from an unidentified strain with 0.1 pg cDNA. We also found cell size and cDNA to be significantly related. We applied the traditional identification keys to some strains and discussed the many problems associated with. A critical revision needs to be done, also because the species probably were described to narrow resulting in a huge number of species of more than 400. To get a first insight into phylogenetic relationships, we conducted molecular analyses of the SSU rDNA. Spirogyra is positioned ancestral to the remaining Zygnematales. As Spirogyra can easily be recognized by its simple vegetative morphology, a low genetic diversity might be deduced, which is however not true; our results revealed an incredibly high variability of the SSU rDNA sequences, a high evolutionary rate and a long evolutionary history. This was also fortified by a secondary loss of the so-called 1506- intron. Interestingly, the Spirogyra introns exhibit features both of early desmids and of later diverging desmids. The answer to the problem of species delineation is certainly not easy; it will take a lot of time it will need more than a single perspective to find a solution. In this regard, the family of Zygnemataceae in general and the genus Spirogyra in particular definitely will continue to be a challenging task of my working group.