The Life Cycle of Chara, a Fresh Water Green Alga

Описание: This animated short film is a look at the life history of Chara, a close relative of embryophytes (land plants). A life history is the series of growing and reproductive changes an organism undergoes throughout its life. Chara is a genus in the fresh water green algal family Characeae, commonly called stoneworts or musk grasses. For this video, the morphology of Chara rusbyana was studied by using a living culture and by consulting the botanical literature. A key difference between the life history of land plants and the Characeae is that alternation of generations is found in land plants but not in the Characeae. Given that the Characeae are close relatives of land plants, understanding their life history is valuable to our ever-broadening knowledge of the early evolution of land plants.

Narration Text:

In this freshwater pond, a close cousin of land plants is about to begin its life cycle. It is in the family of algae called the Characeae.

We will begin with the zygospore, a dormant zygote deposited here by its mother plant previous year.

The single cell within contains starchy nutrients from its mother that feeds its infantile growth. Before dividing even once, the diploid zygote undergoes meiosis… or so we think, as this has yet to be confirmed.

Humans are diploid. Meiosis occurs before fertilization in order to create haploid gametes from our diploid genome, forming a diploid zygote that then undergoes mitosis.

Chara rusbyana is haploid. Meiosis occurs after fertilization in order to create its haploid genome from a diploid zygote, formed by haploid gametes that were created by mitosis.

Either way, this mixes the parents’ genomes and creates completely new gene combinations in the offspring – the evolutionary benefit of sexual reproduction.

What emerges from the top of the spore coat is a single haploid cell. At its first division, the larger cell becomes the root-like rhizoid and the smaller cell the green photosynthetic filament.

The green filament bulges and first grows a whorl of five simple branchlets. The apex continues to grow until another whorl appears with yet another contained within it, and another… and another.

After the first whorl of 5 branchlets, most of the plant body is covered by rows of cells forming a protective cortex.

Eventually, some nodes will grow axillary buds that become new apices and branch off of the main plant body.

Some Characeae, such as this Chara rusbyana grow more adventurous branches that extend centimeters from the plant. These Grovesian Branches propagate the plant.

Over the course of the season the alga well establishes itself.

Our plant happens to be a male. Fortunately a female seems to be growing nearby.

And by now they have grown near enough to have sex.

We know our plant is male by its orange sperm-producing organs called Antheridia.

Inside of this Antheridium, filaments of cells propagate by mitosis – their cytoplasm becoming sperm, which swim free upon maturity.

The 8 scutum cells then open to release the batch into the water.

The female alga has also been busy growing its egg cell structures, the Oogonia. The Oogonium consists of a large egg cell surrounded five twisted cortex cells and five tip cells at the top entrance.

A single sperm finds its way to the top of the oogonium where it swims in to fertilize the egg, creating a diploid zygote. The Oogonium stores starch around the zygote and forms a calcified shell for protection. The zygospore then falls from the mother plant to the pond floor awaiting the Spring.

It will then undergo meiosis to mix its parents’ genes and keep one of the four mixed copies for itself and repopulate this pond once again in the spring.