In zoology, spores are structures that are used by organisms to survive a period of unfavorable environmental conditions, and can subsequently regenerate into the adult form once the environment again becomes favorable for growth. Depending on the species, spores are asexual, resting bodies, which can be one-celled or multi-cellular.
Many protozoans have a stage in their life cycle that involves the development of a spore or cyst that is capable of surviving a period of environmental conditions that are unfavorable for growth. This is especially common among parasitic protozoans, which must survive the unfavorable conditions that are encountered during transmission from host to host, often through the ambient environment. Other, free-living protozoans commonly develop a spore stage to survive periods of severe environmental stress, for example, when a pond dries up during late summer or freezes during winter.
Many types of bacteria, fungi, actinomycetes, yeasts, and algae also develop spores as resting stages to survive periods of unfavorable environmental conditions. A commonly known spore-forming bacterium is Bacillus anthracis, the bacterium that causes anthrax. Anthrax spores can survive in soil for several years, and pose a threat mainly to animals and livestock.
In addition, most fungi develop spores as part of their generative process. Fungi produce diploid spores in specialized organs known as sporangia. Fungal spores are capable of developing into a mature organism once favorable environmental conditions are encountered. Fungal spores are extremely light and can be carried for great distances by wind or water, and they are therefore an extremely effective means of long-distance dispersal. Liverworts and mosses also produce small diploid spores as a means of achieving an extensive dispersal of their asexual progeny to colonize new habitats.
In botany, spores are reproductive cells that are capable of developing into a new individual plant, either directly or after fusion with another spore. Plant spores known as gonidia are developed by mitosis, or the process of division and separation of chromosomes that occurs in a dividing cell. Mitosis produces two diploid daughter cells, each with the same chromosomal content as their parent cell. These types of spores are capable of producing a mature organism without undergoing fusion with another type of spore. The diploid spores of club-mosses and ferns, which are vascular plants, are bisexual structures that are used to propagate and disperse the plants.
Plant spores known as meiospores are developed through the process of meiosis. Meiosis refers to reduction division of a diploid cell, which results in the formation of two haploid spores, each of which contains one of the two sets of chromosomes of the parent cell. Vascular plants produce two types of haploid spores. Megaspores are usually larger and are regarded as the female spore in sexual reproduction of plants, because the female gametophyte develops from these types of spores. Microspores are smaller, and they develop into the male gametophyte. Fusion of the male and female gametophytes leads to the development of plant seeds, which are diploid structures that culminate sexual reproduction in higher plants. Seeds can be dispersed into the environment to colonize new habitats and perpetuate the species.
Researching spore development and survival has aided medical science for over a century. In the 1940s, one of the initial obstacles to the mass-manufacture of the antibiotic Penicillin was the environmental sensitivity of its reproductive spores. The development of new methods to culture penicillin in large quantities lead not only to the wide availability of natural penicillin, but also to the creation of synthetic, or lab created, alternative antibiotics. Pollen and mold allergies, including the manner in which some airborne spores produce certain reactions in humans, have been a focus of spore-related research since 1880. Today, they remain a key interest of research and development in the pharmaceutical industry.