Charales

Introduction

The green algae in the order Charales are commonly called stoneworts or brittleworts. These algae are common in quiet freshwater habitats such as ponds and streams; a few are found in brackish water. Stoneworts are found on all continents except Antarctica. The Charales are the only living stoneworts; two orders of extinct algae closely related to the Charales are known only from fossils of their reproductive spores. The thallus of the plant consists of a series of so-called "giant cells" up to several cm in length, with branches coming off at nodes composed of smaller cells. Growth occurs at the apex, and the plant is anchored in mud or silt by translucent rhizoids. Male and female sexual reproductive structures grow at the nodes. The male antheridia are spherical and often orange. The female oogonia are oblong, consisting of a central cell jacketed by five tubular, spiraling cells. A crown of smaller cells sits atop the jacket cells where they come together.

The thallus in some stoneworts is encrusted with white lime, giving a crusty texture (hence the name brittlewort). Often the thallus has a strong unpleasant smell, and the plants are sometimes called skunkweed.

Discussion of Phylogenetic Relationships

Six living genera in a single family, the Characeae are known. Chara and Nitella contain the most species. Conventional taxonomy (Wood and Imahori, 1965) divides the Charales into two tribes: tribe Chareae (Chara, Nitellopsis, Lamprothamnium, Lychnothamnus), and tribe Nitelleae (Nitella, Tolypella). The Chareae have a crown of five cells on the oogonium, whereas the Nitelleae have a crown of ten cells. Recent molecular studies using rbcL sequences (McCourt et al., 1996) supports the Chareae as a monophyletic group (with the topology shown), with Tolypella and Nitella basal to the group However, the rbcL data suggest that the latter two genera form a basal grade rather than a monophyletic tribe:

   
              === Chara
           ===|
           |  === Lamprothamnium
     ======|
     |     |  === Lychnothamnus
=====|     ===|
     |        === Nitellopsis
     |
     |=========== Tolypella
     |
     ============ Nitella 

References

McCourt, R. M., Karol, K. G., Guerlesquin, M., and Feist, M. (1996) Phylogeny of extant genera in the family Characeae (Charales, Charophyceae) based on rbcL sequences and morphology. Amer. J. Bot. (in press)

Wood, R. D., and K. Imahori. (1965) Monograph of the Characeae. First part of a revision of the Characeae. Verlag von J. Cramer, Weinheim.

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