Spores - Reproductive Cells

Spores - Reproductive Cells

Spores are reproductive cells in plants, fungi, algae, and protozoa. They are typically single-celled and have the ability to develop into a new organism. Unlike gametes in sexual reproduction, spores do not need to fuse in order for reproduction to take place. Organisms use spores as a means of asexual reproduction. Spores are also formed in bacteria, however, bacterial spores are not typically involved in reproduction. These spores are dormant and serve a protective role by safeguarding bacteria from extreme environmental conditions.

Bacterial Spores

Streptomyces Bacterial Spores
This is a colored scanning electron micrograph (SEM) of chains of spores of the soil bacteria Streptomyces. The bacteria commonly grow in the soil as branching networks of filaments and chains of spores (as seen here). Credit: MICROFIELD SCIENTIFIC LTD/Science Photo Library/Getty Images

Some bacteria form spores called endospores as a means to combat extreme conditions in the environment that threaten their survival. These conditions include high temperatures, dryness, the presence of toxic enzymes or chemicals, and lack of food. Spore-forming bacteria develop a thick cell wall that is waterproof and protects bacterial DNA from desiccation and damage. Endospores can survive for long periods of time until conditions change and become suitable for germination. Examples of bacteria that are capable of forming endospores include Clostridium and Bacillus.

Algal Spores

Chlamydomanas Green Algae
Chlamydomanas reinhardtii is a a type of green algae that reproduces asexually by producing zoospores and aplanospores. These algae are also capable of sexual reproduction. Dartmouth Electron Microscope Facility, Dartmouth College (Public Domain Image)

Algae produce spores as a means of asexual reproduction. These spores may be non-motile (aplanospores) or they may be motile (zoospores) and move from one place to another using flagella. Some algae can reproduce either asexually or sexually. When conditions are favorable, the mature algae divide and produce spores that develop into new individuals. The spores are haploid and are produced by mitosis. During times when conditions are unfavorable for development, the algae undergo sexual reproduction to produce gametes. These sex cells fuse to become a diploid zygospore. The zygospore will remain dormant until conditions become favorable once again. At such time, the zygospore will undergo meiosis to produce haploid spores.

Some algae have a life cycle that alternates between distinct periods of asexual and sexual reproduction. This type of life cycle is called alternation of generations and it consists of a haploid phase and a diploid phase. In the haploid phase, a structure called a gametophyte produces male and female gametes. The fusion of these gametes forms a zygote. In the diploid phase, the zygote develops into a diploid structure called a sporophyte. The sporophyte produces haploid spores via meiosis.

Fungal Spores

Puffball Fungus Spores
This is a colored scanning electron micrograph (SEM) of puffball fungus spores. These are the reproductive cells of the fungus. Credit: Steve Gschmeissner/Science Photo Library/Getty Images

Most spores generated by fungi serve two main purposes: reproduction through dispersal and survival via dormancy. Fungal spores can be single-celled or multicelluar. They come in a variety of colors, shapes, and sizes depending on the species. Fungal spores can be asexual or sexual. Asexual spores, such as sporangiospores, are produced and held within structures called sporangia.  Other asexual spores, such as conidia, are produced on filamentous structures called hyphae. Sexual spores include ascospores, basidiospores, and zygospores. Most fungi rely on the wind to disperse spores to areas where they can germinate successfully. The spores can be actively ejected from reproductive structures (ballistospores) or can be released without being actively ejected (statismospores). Once in the air, the spores are carried by the wind to other locations. Alternation of generations is common among fungi. Sometimes environmental conditions are such that it is necessary that fungal spores go dormant. Germination after periods of dormancy in some fungi can be triggered by factors including temperature, moisture levels, and the numbers of other spores in an area. Dormancy allows fungi to survive under stressful conditions.

Plant Spores

Fern Sporangia
This fern leaf has sori or fruit dots, which contain clusters of sporangia. Sporangia produce plant spores. Credit: Matt Meadows/Photolibrary/Getty Images

Like algae and fungi, plants also exhibit alternation of generations. Plants without seeds, such as ferns and mosses, develop from spores. Spores are produced within sporangia and are released into the environment. The primary phase of the plant life cycle for non-vascular plants, such as mosses, is the gametophyte generation (sexual phase). The gametophyte phase consists of green mossy vegetation, while the sporophtye phase (nonsexual phase) consists of elongated stalks with spores enclosed within sporangia located at the tip of the stalks. In vascular plants that do not produce seeds, such as ferns, the sporophtye and gametophyte generations are independent. The fern leaf or frond represents the mature diploid sporophyte, while the sporangia on the underside of the fronds produce spores that develop into the haploid gametophyte.

In flowering plants (angiosperms) and nonflowering seed-bearing plants, the gametophyte generation is totally dependent upon the dominant sporophtye generation for survival. In angiosperms, the flower produces both male microspores and female megaspores. The male microspores are contained within pollen and the female megaspores are produced within the flower ovary. Upon pollination, the microspores and megaspores unite to form seeds, while the ovary develops into fruit.