An overview of Nitella (Characeae, Charophyceae) in Australia
Michelle T. CasanovaRoyal Botanic Gardens Melbourne, Birdwood Avenue, South Yarra, Vic. 3141, Australia and 273 Casanova Road, Westmere, Vic. 3351, Australia. Email: amcnova@netconnect.com.au
Australian Systematic Botany 22(3) 193-218 https://doi.org/10.1071/SB08039
Submitted: 25 August 2008 Accepted: 12 March 2009 Published: 10 June 2009
Abstract
The genus Nitella Ag. in algal family Characeae is characterised by furcate (forked) branchlets, compressed oospores (i.e. oval in cross-section), terminal antheridia and a 10-celled coronula on the oogonium. Species of Nitella are submerged plants that grow in a variety of wetland and riverine habitats. Approximately 89 taxa of Nitella (species, subspecies, varieties and forms) have been described on the basis of Australian collections, and published estimates of the number of species range from 18 to >35. The lower value is based on the assumption that infra-specific variation is great, species have a wide distribution, monoecy and dioecy are not indicative of speciation and the number of furcations and the ratio of branchlet segment lengths vary for a species owing to the environment in which they grow. The higher value is based on evidence that morphological characters are relatively constant for a species, that oospore variation is a good indication of speciation and that monoecious and dioecious entities are not inter-fertile. An overview of Australian members of the genus is presented here as a framework for further taxonomic work. Representatives of all three subgenera of Nitella occur in Australia, with subgenus Nitella poorly represented, and subgenera Tieffallenia and Hyella equally speciose. The subgenera are defined here in relation to the Australian taxa they contain. In the present treatment, section Migularia is transferred from subgenus Tieffallenia to subgenus Hyella, and several species are transferred to subgenus Tieffallenia. Within subgenus Tieffallenia, variation in vegetative and oospore morphology is useful for distinguishing among sections and species. However, although members of subgenus Hyella display a similar range of variation in vegetative morphology, most of the species have similar, reticulate, oospore ornamentation. Australia is home to a large number of endemic species of Nitella, many of which are dioecious. The total number of species and the degree of endemism have been underestimated in earlier studies, and it is likely that more than 50 species of Nitella will be recognised on the basis of Australian specimens. A key to the subgenera, and keys to sections in the subgenera are provided.
Acknowledgements
Staff at the Royal Botanic Gardens Melbourne (National Herbarium of Victoria) and the State Herbarium of South Australia have facilitated this study in many ways. This study would have been much more difficult (if not impossible) without the texts and translations of very early references given to me by Beth Williams, thank you Beth. Discussions with and data provided by Dr Ken Karol about his work on nucleotide sequences in Nitella have been important in the development of this paper. Oospore micrographs were taken at the Scanning Electron Microscope Unit of the University of Melbourne with the assistance of Dr Simon Crawford. This work has been funded, in part, by Australian Biological Resources Study Grants to Dr Gerry Kraft and M. T. Casanova in 2004, and to M. T. Casanova in 2007–2009. Drs John Porter, Margaret Brock and Tony Dugdale, Keely Ough, Hans and Annie Wapstra have provided beautiful specimens of Nitella that have greatly enhanced this research.
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