Meiosis occurs in what types of organisms

Meiosis is a specialized form of cell division that produces reproductive cells, such as plant and fungal spores and sperm and egg cells. In general, this process involves a "parent" cell splitting into two or more "daughter" cells. In this way, the parent cell can pass on its genetic material from generation to generation.

Based on the relative complexity of their cells, all living organisms are broadly classified as either prokaryotes or eukaryotes. Prokaryotes, such as bacteria , consist of a single cell with a simple internal structure. Their DNA floats freely within the cell in a twisted, thread-like mass called the nucleoid.

Animals, plants and fungi are all eukaryotes. Eukaryotic cells have specialized components called organelles, such as mitochondria , chloroplasts and the endoplasmic reticulum.

Each of these performs a specific function. Unlike prokaryotes, eukaryotic DNA is packed within a central compartment called the nucleus. Within the eukaryotic nucleus, long double-helical strands of DNA are wrapped tightly around proteins called histones. This forms a rod-like structure called the chromosome. Cells in the human body have 23 pairs of chromosomes, or 46 in total.

This includes two sex chromosomes: two X chromosomes for females and one X and one Y chromosome for males. Because each chromosome has a pair, these cells are called "diploid" cells. On the other hand, human sperm and egg cells have only 23 chromosomes, or half the chromosomes of a diploid cell. Thus, they are called "haploid" cells.

When the sperm and egg combine during fertilization, the total chromosome number is restored. That's because sexually reproducing organisms receive a set of chromosomes from each parent: a maternal and paternal set. Each chromosome has a corresponding pair, orhomolog. By contrast, in all land plants mitotic divisions intercede fertilization and meiosis, and meiosis occurs after a period of diploid development in specialized structures termed sporangia that produce numerous spores Bower, ; Becker and Marin, The initiation of sporangium development pathways often follows a switch in meristem identity from a vegetative to a reproductive fate Steeves and Sussex, The location and structure of sporangia vary by plant group and are associated with their secondary functions, which are spore dispersal and nutrition.

Heteromorphic sporangia and spores have evolved convergently in vascular plants and associate with specialized functions Fig. Female megasporangia have fewer, larger spores megaspores that may be retained within the parent plant after fertilization, whereas male microsporangia develop numerous small spores microspores that have a dispersal function Bower, Gender can also influence patterns of CO frequency Drouaud et al. Thus the initiation and progression of meiosis depend on the developmental identity of the tissue in which it is activated, discussed by plant group below.

Phylogenetic distribution of characters associated with meiosis in plants. A Phylogenetic relationships between major plant groups showing synapomorphies associated with meiosis. Heterospory has a polyphyletic origin in lycophytes, monilophytes, and spermatophytes, indicated by asterisks. B Reproductive structures of representatives of clades illustrated in A , and extinct protracheophyte fossil forms. The zygospore wall of Chlamydomonas monoica Chlorophyceae : Morphogenesis and evidence for the presence of sporopollenin1.

Journal of Phycology 33, — Light micrograph of the haploid plant and oogonium inset in the charophyte alga Chara sp. The creeping haploid thallus and a diploid erect and determinate sporophyte of Pellia epiphylla photographs of bryophytes courtesy of Li Zhang. The haploid leafy gametophyte of Atrichum angustatum bearing diploid unbranched determinate sporophytes.

The haploid creeping thallus of Folioceros sp. A fossil sporophtye of Cooksonia sp. A fossil sporophyte of Zosterophyllum showing lateral sporangia photo courtesy of Jenny Morris and Dianne Edwards. A Selaginella kraussiana sporophyte showing vegetative branching habit, an unbranched reproductive strobilus, and a mega- and microsporangium inset. Sporangia formed on the stem of Psilotum nudum.

A frond of Adiantum mairisi showing marginal sori that contain sporangia. The terminal cones of Cupressus sp. The stamens and carpels of a Magnolia sp. In both chlorophyte and charophyte algal sister groups to the land plants meiosis occurs immediately following fertilization Becker and Marin, In the single-celled chlorophyte alga, Chlamydomonas reinhardtii two haploid mating types, plus and minus , differentiate into gametes which fuse during fertilization to form a single-celled zygote Fig.

Following fertilization GSP1 and GSM1 heterodimerize, translocate to the nucleus, and initiate zygotic gene expression patterns Lee et al. Constitutive expression of either GSP1 or GSM1 in the opposite gamete type is sufficient to trigger zygote development in the absence of fertilization Zhao et al. Stable C. In contrast to chlorophytes, charophytes have a multicellular haploid body that generates free-swimming sperm in antheridia and egg cells that are retained within an oogonium on the parent plant Fig.

Egg retention oogamy is an innovation shared with land plants thought to have been a key adaptation in their evolution McCourt et al. In contrast to their algal sisters, all land plants have a period of multicellular diploid growth, the extent of which varies by plant group Lewis and McCourt, ; McCourt et al. The bryophyte sister groups to the vascular plants exhibit limited post-embryonic development with no indeterminate apical growth Mishler and Churchill, ; Shaw and Renzaglia, ; Donoghue, Sporophytes comprise a small single stem with a terminal sporangium that represents the simplest basal land plant body plan Kenrick, ; Donoghue, ; Qiu et al.

In liverworts and mosses sporangium development arrests diploid growth, whereas hornwort sporophytes contribute to their own nutrition and have sporangia that grow indeterminately from a basal meristem Boyce, ; Kato and Akiyama, A sub-epidermal archesporial cell layer is specified during sporangium development and divides either by meiosis to generate spores mosses or spore mother cells and interspersed elater cells that perform nutritive or dispersal functions liverworts and hornworts.

The tissues surrounding the archesporial cell layer perform dispersal functions specific to each bryophyte group Bower, The genetic and developmental mechanisms that regulate bryophyte sporophyte development are currently poorly understood, but interest has recently accelerated due to the establishment of moss Physcomitrella patens and liverwort Marchantia polymorpha models Ishizaki et al. Two gene classes that affect sporangium development in P.

A pair of LFY homologues redundantly control the first zygotic division in P. In mutants that do not arrest, sporangium number, initiation, and development are perturbed. These defects may arise as a consequence of abnormal sporophytic development, although spore number and germination are also highly variable in the mutants, suggesting meiotic defects Tanahashi et al.

Key features that distinguish vascular plants from bryophytes are the elaboration of an indeterminately growing and branching diploid body Mishler and Churchill, ; Donoghue, ; Langdale and Harrison, Fossil plants whose form is not represented in living plants, such as Cooksonia , have low orders of branching and may have amplified spore numbers by increasing numbers of terminal sporangia Fig. These fossils raise interesting questions about the developmental nature of the association between axis development, sporangium development, and branching and, intriguingly, rare bryophyte branching mutants strikingly resemble Cooksonia sporophytes Fig.

Alternative lateral sporangial placements appear independent of branching and may have served a similar purpose in other fossil groups Fig. This arrangement is exhibited in modern lycophytes, and sporangia arise either at the base of leaves or from the stem via one or two sub-epidermal archesporial cell layers. These archesporial cells give rise to sporogenous tissue Lycopodium or sporogenous and tapetal tissues Selaginella , Isoetes Bower, Monilophyte sporangia are diverse in terms of their size, the number of spores produced per sporangium, and their number and position on the plant Fig.

The eusporangiate basal monilophyte grade comprising marattioid ferns, horsetails, ophioglossoid ferns, and whisk ferns possess sporangia that develop from several cells and produce thousands of spores Bower, ; Wagner, ; Parkinson, ; Pryer et al. By contrast, the leptosporangiate ferns develop numerous, small sporangia from single cells, which typically contain tens of spores Bower, ; Pryer et al.

Sporangia may show terminal, adaxial, abaxial, or marginal locations on leaves Fig. With the exception of the leptosporangiate and whisk ferns, nutritive tapetal tissues arise from non-sporogenous tissue Bower, ; Parkinson, As in bryophytes, the genetic basis of diploid development is poorly characterized in lycophytes and monilophytes.

Notably sporophytic KNOX expression is conserved, and meristematic expression domains suggest likely roles in indeterminate growth Bharathan et al. Thus the structure and dispersal functions of sporangia vary broadly across the land plants and the developmental context for the initiation of meiosis is lineage specific. An evolutionary trend towards the amplification of spore numbers by alterations in body plan, sporangium size, and the number of sporangia is apparent Bower, In seed plants gymnosperms and angiosperms a prolonged period of vegetative growth is followed by the reproductive transition.

This transition involves a change in meristem identity and leads to the development of cones or flowers Steeves and Sussex, Seeds develop in the context of the ovule following fertilization of the female egg cell by a male sperm cell transferred in pollen, thus dispersal functions are provided both by haploid pollen and diploid seed.

Ovules are the site of megasporangium nucellus development, which precedes meiosis. Whilst in gymnosperms one to several nucellar cells enter meiosis, in angiosperms a single megaspore mother cell undergoes meiosis to form a tetrad, three members of which degenerate to form a single functional megaspore, which divides mitotically to form the embryo sac Campbell, ; Colombo et al. Pollen sac microsporangium development occurs from a microsporophyll or in the anther in gymnosperms and angiosperms respectively.

In both, sub-epidermal cells are specified as archesporial cells that divide periclinally to form a layer of parietal cells surrounding the sporogenous cells Campbell, ; Feng and Dickinson, Sporogenous cells may then either directly enter meiosis or continue to proliferate.

Parietal cells divide further to form a variable number of concentrically arranged cell layers, the innermost of which differentiates into the nutritive tapetum Campbell, ; Feng and Dickinson, Callose appears to play an important role in sporogenesis, as tapetal expression of callase causes male sterility in tobacco Worrall et al.

Following meiosis, the resulting haploid microspores undergo mitosis and differentiate into pollen grains. The genetic control of vegetative development, the reproductive transition, and sporangium formation are well studied in the angiosperm A. Thus, in A. Flower development follows conversion of indeterminate, vegetative shoot meristems to reproductive fates. This switch is controlled by a large network of genes that ensure reproduction is co-ordinated with environmental and developmental conditions Baurle and Dean, Floral organ identity genes encode three functional classes of MADS-box transcription factors A, B, and C that are expressed in overlapping domains to specify the four floral organ types Coen and Meyerowitz, Thus genes involved in meristem identity also play roles in the specification of reproductive fate.

In both micro- and megasporangia archesporial cell specification precedes sporangium formation Gifford and Foster, Ectopic activation of SPL in agamous mutants is sufficient to induce staminoid development and pollen formation Ito et al. Male sporocyte identity in A. The first archesporial cell division normally separates reproductive sporocyte fate from non-reproductive wall and tapetal fates.

Interestingly, additional LRR receptor kinases have also been implicated in proper differentiation of the anther cell layers Albrecht et al. How these signalling processes are organized between the cell types within the developing anther is not yet clear.

Researchers' initial understanding of meiosis was based upon careful observations of chromosome behavior using light microscopes. Then, in the s, electron microscopy provided scientists with a glimpse of the intricate structures formed when chromosomes recombine. More recently, researchers have been able to identify some of the molecular players in meiosis from biochemical analyses of meiotic chromosomes and from genetic studies of meiosis-specific mutants.

Meiosis represents a survival mechanism for some simple eukaryotes such as yeast. When conditions are favorable, yeast reproduce asexually by mitosis. When nutrients become limited, however, yeast enter meiosis.

The commitment to meiosis enhances the probability that the next generation will survive, because genetic recombination during meiosis generates four reproductive spores per cell, each of which has a novel genotype.

The entry of yeast into meiosis is a highly regulated process that involves significant changes in gene transcription Lopez-Maury et al. By analyzing yeast mutants that are unable to complete the various events of meiosis, investigators have been able to identify some of the molecules involved in this complex process. These controls have been strongly conserved during evolution , so such yeast experiments have provided valuable insight into meiosis in multicellular organisms as well. In most multicellular organisms, meiosis is restricted to germ cells that are set aside in early development.

The germ cells reside in specialized environments provided by the gonads, or sex organs. Within the gonads, the germ cells proliferate by mitosis until they receive the right signals to enter meiosis. In mammals, the timing of meiosis differs greatly between males and females Figure 2. Male germ cells, or spermatogonia, do not enter meiosis until after puberty. Even then, only limited numbers of spermatogonia enter meiosis at any one time, such that adult males retain a pool of actively dividing spermatogonia that acts as a stem cell population.

On the other hand, meiosis occurs with quite different kinetics in mammalian females. Female germ cells, or oogonia, stop dividing and enter meiosis within the fetal ovary.

Those germ cells that enter meiosis become oocytes, the source of future eggs. Consequently, females are born with a finite number of oocytes arrested in the first meiotic prophase.

Within the ovary, these oocytes grow within follicle structures containing large numbers of support cells. The oocytes will reenter meiosis only when they are ovulated in response to hormones. Human females, for example, are born with hundreds of thousands of oocytes that remain arrested in the first meiotic prophase for decades. Over time, the quality of the oocytes may deteriorate; indeed, researchers know that many oocytes die and disappear from ovaries in a process known as atresia.

Two divisions, meiosis I and meiosis II , are required to produce gametes Figure 3. Meiosis I is a unique cell division that occurs only in germ cells; meiosis II is similar to a mitotic division. Before germ cells enter meiosis, they are generally diploid , meaning that they have two homologous copies of each chromosome. Then, just before a germ cell enters meiosis, it duplicates its DNA so that the cell contains four DNA copies distributed between two pairs of homologous chromosomes.

Compared to mitosis, which can take place in a matter of minutes, meiosis is a slow process, largely because of the time that the cell spends in prophase I. During prophase I, the pairs of homologous chromosomes come together to form a tetrad or bivalent , which contains four chromatids. Recombination can occur between any two chromatids within this tetrad structure. The recombination process is discussed in greater detail later in this article.

Crossovers between homologous chromatids can be visualized in structures known as chiasmata, which appear late in prophase I Figure 4.

Chiasmata are essential for accurate meioses. At the end of prometaphase I, meiotic cells enter metaphase I. Here, in sharp contrast to mitosis, pairs of homologous chromosomes line up opposite each other on the metaphase plate , with the kinetochores on sister chromatids facing the same pole.

Pairs of sex chromosomes also align on the metaphase plate. In human males, the Y chromosome pairs and crosses over with the X chromosome. These crossovers are possible because the X and Y chromosomes have small regions of similarity near their tips.

Crossover between these homologous regions ensures that the sex chromosomes will segregate properly when the cell divides.

Next, during anaphase I , the pairs of homologous chromosomes separate to different daughter cells. Before the pairs can separate, however, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter cells is referred to as nondisjunction , and it is a major source of aneuploidy. Overall, aneuploidy appears to be a relatively frequent event in humans.

Meiosis II resembles a mitotic division, except that the chromosome number has been reduced by half. Thus, the products of meiosis II are four haploid cells that contain a single copy of each chromosome. There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes.

Reference Terms. In other words, meiosis and sexual reproduction produce genetic variation. Related Stories. For almost 15 years, it has been commonly held that retinoic acid, a molecule derived from vitamin A, triggers meiosis in mammalian germ cells. Yet new The incorporation of Even those who study it have used the word 'wimpy' to describe it, and yet it continues to stick around. Haploid cells are a powerful genetic tool to analyse gene function. In the past decade, a number