Home / Charophyceae / Desmids
"Desmids" (desmos + eidos = chain forming) denotes the group of green protists that reproduce sexually with amoeboid gametes (without flagella), either by forming "conjugation bridges" through which the "male" gamete migrates (filamentous forms) or by tangential contact within a common mucilage sheath, also with gamete migration, then fusing nuclei to form a diploid zygote. They can also reproduce asexually by simple cell division (saccoderm desmids)(Jarman and Pickett-Heaps 1990) or by budding (placoderm desmids). The saccoderms (saccus = sack) have one entire cellulose outer wall. The placoderms (plax = flat round plate or tablet) have a thin or thick isthmus dividing two hemicells, and two cellulose wall halves that overlap in the region of the isthmus. Cell reproduction involves separation of the two hemicells followed by nuclear migration (sexually during conjugation of two cells) or budding (asexual) followed by growth to full hemicell size for the species. In phycokey the desmids are placed in the Charophyceae (no longer in the Chlorophyceae) and are separated as unicells (both saccoderms and placoderms) and filaments (only placoderms). Even this revision has been dropped by some phylogenists on genetic bases. Classic identification, description and phylogenetic relationships of desmids has led to ~4,000 species descriptions (Gontcharov 2013), many of which may be ecads or growth forms. Dabbling with Staurastrum cultures as a graduate student convinced me that growth ‘conditions’ can produce new morphotypes, some asymmetric. What value justifies classic taxonomy if any desmid ‘species’ can generate polymorphs?  Do any desmids hybridize, or just produce polymorphs? A phylogeny so based would be artificial and poorly related to genetic relationships. “Genera and species of the Zygnemophyceae are mostly based on differences in cell organization (filaments/unicells), cell symmetry, size and shape, cell wall projections and ornamentations and chloroplast structure. In most taxa neither the stability of the taxonomic characters nor their evolutionary status (plesiomorphic or apomorphic) have been studied in any detail” (Gontcharov 2013). Alan Brook devoted most of his academic life to studying the morphology and ecology of desmids. Photo credit: Obituary (John and Whitton 2013). Alan J. Brook (1923 – 2013)
References:
Gontcharov, A. 2013. Molecular phylogeny of the conjugating green algae (Zygnemophyceae, Streptophyta). [online] Jarman, M., and J.Pickett-Heaps 1990. Cell division and nuclear movement in the saccoderm desmid Netrium interruptus. Protoplasma 157:136-143. [Online] John, D.M., and B.A. Whitton 2013.  Obituary.  Phycologist 85: 46-47. [online]