The process of sexual reproduction

Essentially, desmids are haploid organisms (so every gene is but present in singular). Mating of  desmid cells (by means of conjugation) results in a diploid zygospore that, after a shorter or longer time, undergoes a meiotic cell division resulting in a number of haploid germlings. Those germlings may, by mitotic division, build up a new population of vegetative cells.

Usually, sexually active cells are morphologically hardly or not to be distinguished from vegetative cells, but they differ from them by producing hormones that trigger mating behaviour. The existence of such interacting hormones is particularly evident in species in which sexual cells do not mate randomly but only in combinations of physiologically different cells, called ‘plus’ (+) and ‘minus’ (-) cells. 

Interaction of hormones produced by + and – cells results in mutual attraction and subsequent chemotaxis. Movement of sexual active cells (by means of directional excretion of mucous through given cell wall pores) is not fast but nevertheless may break substantial barriers, as is illustrated by the herewith reproduced images of Closterium ehrenbergii cells.

migration of mating cells of Closterium ehrenbergii

Left:

An agar strip (ca 5 mm in breadth) separates suspensions of + (left) and – (right) cells of Closterium ehrenbergii.  + Cells are penetrating the agar strip, moving towards the – cells.

Right: Idem, in detail.

(more information in Coesel & De Jong, 1986)


images  © Peter Coesel


When partner cells have approached each other closely enough, the process of conjugation may evolve:  partner cells functioning as gametangia break open (usually at the isthmus), protoplasts functioning as gametes move out and mutually fuse to a diploid zygote. The zygote develops a thick, resistant wall which (dependent on the species) may be furnished with projections like tubercles or spines. Semicell walls of the empty gametangia often adhere the zygospore for some time.

Smooth-walled zygospore of Cosmarium reniforme with adhering empty gametangial cells.

Spiny zygospore of Cosmarium botrytis with adhering empty gametangial cells.

both images © Peter Coesel

Detailed sexual cycles, at best to be studied in culture, are known from but a few species (see also Brook, 1981).

One of the species studied is Closterium ehrenbergii, characterized by the formation of twin zygospores. Twin zygospores are formed when partner cells undergo cell division immediately before conjugation so that actually four cells, lieing close together, participate in the mating process. 
  1. Pairing of + and – cells of Closterium ehrenbergii
  1. Each partner cell has divided. Young semicells do not grow out but make mutually (+ and -) contact
  1. Detail of starting conjugation: papillae arise from the conically shaped young semicell and a hyaline, interconnecting conjugation vesicle is secreted
  1. Fusion of + and – gametes within conjugation vesicles.
  1. Formation of smooth-walled twin zygospores (each zygospore with two adhering gametangial cells)
  1. Zygospore after having been stored under dry conditions in the dark for some months. Two chloroplast residues can be recognized.

  1. Two daughter cells (germlings) clearly visible within the zygospore wall after rewetting (after meiosis, two of the potentially four haploid nuclei degenerate).

  1. Developing germlings within a vesicle (hardly visible in this picture) that was released through a fissure in the zygopore wall.

  1. Germlings escaping from the germination vesicle. Each germling will give rise, by mitotic cell divisions, to  a new population of vegetative cells.

all images © Peter Coesel


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references:
  • Brook, A.J., 1981. The Biology of Desmids. – Blackwell Scientific Publications, Oxford.

  • Coesel, P.F.M. & W. de Jong, 1986. Vigorous chemotactic attraction as a sexual response in Closterium ehrenbergii Meneghini (Desmidiaceae, Chlorophyta). – Phycologia 25: 405-408.