Carpospores in Chains (Taxon of the Week: Schizoserideae)

Drachiella spectabilis, one of the few species of Schizoserideae found in the North Atlantic. Note the characteristic iridescence of the fronds. If you look very closely at the photo, you can also see one other characteristic of the family Delesseriaceae to which this species belongs - the fronds are so thin that you can see the features of the rock the alga is growing on through them. Photo by Keith Hiscock.

Despite being the most speciose clade of multicellular marine algae, I must admit I find that the Macrorhodophytina* (multicellular red algae) are not that easy to get a handle on. Most of the significant distinguishing features of various groups of red algae are at the cellular level, and often wrapped up in the eye-wateringly complicated life cycles many macrorhodophytes indulge in. So before I wrote this post, I had to spend a certain amount of time looking up things like just what a "gonimoblast" is. I hope I got it right.

*If you're wondering why I didn't use the name Rhodophyta, that's because Rhodophyta is a larger clade that also includes a few unicellular forms.

The Schizoserideae are a tribe of the red algal family Delesseriaceae containing five genera - Schizoseris, Neuroglossum, Abroteia, Drachiella (Lin et al., 2002) and the recently described Nancythalia (Millar et al., 2002). Delesseriaceae is a large family of red algae with very thin fronds (sometimes only a single cell thick) that may be anything from flat, broad and unbranched to very feathery; however the fronds are not filamentous or polysiphonous (a tubular construstion with a central axial cell surrounded by pericentral cells), distinguishing Delesseriaceae from other families in the order Ceramiales. Ceramiales are in turn distinguished from other red algal orders by the mode of formation of the auxiliary cell. To explain what an auxiliary cell is, I have to tell you that Ceramiales, like many other red algae, alternate between not just two but three distinct generations. As well as having separate multicellular haploid and diploid generations (as also found in many other algae and plants), there is a third stage called the carposporophyte. Mature diploids produce haploid spores that settle and grow into mature male or female haploids. The male haploids release sperm that fertilise the females. However, the resulting zygotes are not released; instead, the diploid nucleus of the zygote abandons the zygote and invades a nearby cell to produce the auxiliary cell (in Ceramiales, the cell that becomes the auxiliary was previously one of the supporting cells for the female gamete). The auxiliary cell then gives rise to a small diploid that remains parasitic on the parent haploid - this is the carposporophyte. The carposporophyte produces diploid spores that grow into new independent diploids.


Mature carposporophyte of Schizoseris condensata, showing the large, branching central fusion cell. Figure from Hommersand & Fredericq (1997).

In members of the Schizoserideae, the female gametangium (the procarp) contains four cells called carpogonia, one of which will get fertilised by the sperm, as well as one or two basal and one or two lateral sterile cells. After the fertilised zygote nucleus has entered the auxiliary cell, the carpogonial cells fuse to form a (wait for it) fusion cell. The auxiliary cell then gives rise to the filaments of the carposporophyte (these are the gonimoblast filaments, in case you were still wondering what that was), which in turn produce the carpospores (diploid spores) in long chains. After forming the carpospore chains, the gonimoblast cells then also fuse with the fusion cell, which ends up being a large, candelabra-shaped supportive structure for the carpospore chains; this candelabra-shaped fusion cell is one of the distinguishing characters for the Schizoserideae* (Hommersand & Fredericq, 1997). Other distinguishing features include the lack of protective covering cells on the procarps, and the arrangement of cell nuclei within the fronds - in growing parts of the fronds, all the nuclei line up in a single plane. As well as the morphological characteristics, the tribe has also been supported by molecular analysis (Lin et al., 2001).

*And just to show how much I do for you - those four sentences were the result of probably about an hour of me reading and re-reading the original paper trying to work out just what the heck was going on.


Sections through fronds of Schizoseris condensata, showing the linear arrangement of nuclei. In the bottom section, the alignment is breaking apart in the older part of the frond to the right. Figures from Hommersand & Fredericq (1997).

The main centre of distribution for the Schizoserideae is in the Southern Hemisphere; Abroteia and Nancythalia are both (as far as is known) monotypic and endemic to New Zealand (Millar et al., 2002). The genus Drachiella is the exception, with three of its four species found in the northern Atlantic and the fourth species described only recently from Taiwan and the Philippines (Lin et al., 2002).

REFERENCES

Hommersand, M. H., & S. Fredericq. 1997. Characterization of Schizoseris condensata, Schizoserideae trib. nov. (Delesseriaceae, Rhodophyta). Journal of Phycology 33 (3): 475-490.

Lin, S.-M., S. Fredericq & M. H. Hommersand. 2001. Systematics of the Delesseriaceae (Ceramiales, Rhodophyta) based on large subunit rDNA and rbcL sequences, including the Phycodryoideae, subfam. nov. Journal of Phycology 37: 881-899.

Lin, S.-M., J. E. Lewis & S. Fredericq. 2002. Drachiella liaoii sp. nov., a new member of the Schizoserideae (Delesseriaceae, Rhodophyta) from Taiwan and the Philippines. European Journal of Phycology 37: 93-102.

Millar, A. J. K., & W. A. Nelson. 2002. Nancythalia humilis gen. et sp. nov. and Abroteia suborbiculare (Delesseriaceae, Rhodophyta) from New Zealand. Phycologia 41 (3): 245-253.