What is the significance of sexual reproduction in meiosis

However, each meiotic cell would generate four types of resulting cells after random recombination. If those four cells randomly paired and fused, too much variation would rapidly diversify the characteristic genome structure of the species along with the increase of round from one zygote to zygotes of next generations. This does not even take into account the genetic complexity from a population perspective, in which the meiotically produced cells could pair and fuse with those arising from other meiotic cells.

Is there any way to solve that problem? Heterogametes in animals and plants generally display morphological differences, i. However, as mentioned above, heterogamy in unicellular eukaryotic organisms is frequently determined at the molecular level by a single genetic locus, and can include more than two mating types. This inference is consistent with the recent finding that in Volvox , expansion of a mating locus causes heterogametes to change from being equal in size to being dimorphic Ferris et al.

What is the advantage of heterogamy that enables the mating loci to be selected among the enormous genetic variation? If we remember the problems mentioned above regarding random pairing and fusion of meiotically produced cells in fertilization, we can speculate that heterogamy would significantly restrict diversity.

With heterogamy, meiotically produced cells are classified into different groups that prevent pairing and fusion of those among the same group. As it allows pairing and fusion only between haploid cells from different groups, this harnessing mechanism creates a relatively stable interval during which the adaptive cells can be selected.

If the essential function of heterogamy is the labeling of meiotically produced cells and thereby harnessing variation while enhancing heterogeneity, there should be a multitude of ways to achieve this. Genetic loci for mating types probably represent the most ancient and simple way, but there could be many other modifications to enhance the differentiation for higher efficiency.

In majority of animals familiar to human experience, heterogametes are differentiated from germlines that migrate into and complete the differentiation in dimorphic gonads during embryogenesis. Heterogamy is determined mainly in gonad differentiation prior to germ cells undergoing meiosis.

One may therefore believe that sex determination or differentiation is a precondition of the occurrence of meiosis.

The situation appears similar in plants if we examine only angiosperms. If we compare the divergence points in green algae and the four groups of land plants, we see a trend in which the divergence point s that leads to the heterogamete differentiation shifted from gametophytes after meiosis to sporophytes before meiosis in green algae and angiosperms, respectively.

Little is known regarding how this shift evolved. However, efficiency in gamete distribution and meeting might contribute to the shift: in Chlamydomonas , the two types of gametes differentiate in water and their pairing and fusion occurs randomly. In mosses and ferns, sperm cells are shed into water as well and swim to archegonia and eggs with water as the medium.

In gymnosperms and angiosperms, in which the divergence points are shifted to sporophytes, the delivery of sperm is no longer restricted to water.

This may allow these two groups of plants to increase their spatial distribution. Comparison of life cycles of various autotrophic organisms emphasizing the divergence points resulting in dimorphic structures related to heterogametogenesis.

From left to right: life cycles of selected species representing unicellular green algae Chlamydomonas , multicellular green algae Ulva , mosses Polytrichum , ferns Ploypodium , gymnosperms Pinus , and angiosperms Arabidopsis are briefly outlined. Green arrows indicate morphological transitions in sporophyte generations. Light green arrows indicate morphological transitions in gametophyte generations.

Red triangles indicate the major divergence points leading to dimorphic development for heterogametogenesis. The major divergence points are shifted from post-meiosis in green algae, mosses, and ferns in some species like Ploypodium to before meiosis in gymnosperms and angiosperms.

Reprinted from Bai and Xu , by permission of Elsevier. Similarly, if we examine mechanisms of animal sex differentiation with a broader view, there are also diversifications worth noting: although gametogenesis is carried out in the germlines, sex differentiation mainly occurs at the gonads. While mammalian gonad differentiation from bipotential to unisexual is triggered by sex-determining genes, a similar gonad differentiation is induced by environmental temperature in some reptiles Ramsey and Crews, This implies that over the course of evolution there might be a trend in which determination of heterogametogenesis shifted from germ cells in cis to somatic gonads in trans , and further that the trigger s for gonad differentiation shifted from environmental signals to genetic factors encoded in chromosomes, and even further that the chromosomes bearing genetic factors determining sex evolved into sex chromosomes, as suggested by Charlesworth et al.

If the above speculation is accurate, sex differentiation indeed can be considered essentially a labeling mechanism for heterogamy, regardless of how diversified in form and complicated in regulation, in a wide spectrum of organisms from unicellular eukaryotes to multicellular animals, plants, and fungi.

In animals, meiosis and gametogenesis occur sequentially in germlines and sex differentiation occurs in somatic gonads into which the germline migrated. In plants, meiosis and gametogenesis occur separately from somatic cells of the sporophyte and gametophyte, while sex differentiation could occur either in sporophyte or gametophyte. How were meiosis, gametogenesis, and sex differentiation originally integrated?

Considering that all three processes exist in protists, one possible scenario is that each evolved independently, and they were integrated together as a coordinated process by chance and thereafter genetically fixed as a program in protists. This scenario is possible because all three processes, meiosis, heterogametogenesis including sex differentiation and gametogenesis , and fertilization, occur at the cellular level.

Protists are unicellular eukaryotes and live in a population. These two characteristics provide the required conditions for SRC emergence: on one hand each cell can behave independently for emergence of meiosis and gametogenesis, and on the other, all cells live together closely enough to make both cell fusion and cell—cell recognition possible. Integration of the three events would have brought all of their selective advantages together and such integration, now referred as SRC, would be therefore selected during evolution.

In nearly all biology textbooks, meiosis is introduced in comparison with mitosis, whereas fertilization and sex determination or differentiation are introduced elsewhere.

However, if we view meiosis and fertilization together, we find that one cell becomes four except in some particular cases, such as angiosperms and mammalian, only one female meiotically produced cell remaining alive to differentiate into female gamete through meiosis and two cells become one through fertilization. Thus, the net result of the entire SRC is that one cell becomes two, just like one round of mitosis.

It needs to be emphasized that what can appropriately be compared with the mitotic cell cycle is not meiosis alone, but rather the entire process of SRC, including meiosis, heterogametogenesis, and fertilization. Diagram of the sexual reproduction cycle SRC. A A regular cell cycle for proliferation, through which one cell becomes two, and environmental conditions trigger or affect the cycle at various points in the process.

The net result of the SRC is that one cell becomes two, just as in the regular cell cycle, regardless of how these events evolved and were integrated of which little is known. C Core processes of the life cycles of multicellular organisms.

From the perspective of the SRC, it is clear that all multicellular structures arose in the interval phases of the SRC through the regular cell cycle and cellular differentiation, whether diploid in almost all organisms or haploid mainly in plants and fungi.

Modified from Bai and Xu , by permission of Elsevier. Cell division occurred well before eukaryotes evolved. Despite the difference in complexity between mitotic cell division in eukaryotes and cell fission in prokaryotes, the two processes are similar in that the two resulting cells retain the same genome structure as the starting cell.

By contrast, the genome structures of the two cells resulting from the SRC are no longer the same as that of the original cell, as described above. They are actually clones, the same as in the cell division observed in Escherichia coli leading to a proliferation of the same generation. Only through the SRC is a new generation created. According to Chen et al.

However, only variations retained through the SRC can be maintained from one generation to the next, rather than being diluted and ultimately disappearing through continuous cell divisions. In that sense, mainly because it was integrated as part of the SRC, meiotic recombination took a prominent position among the various ways of creating genetic variations.

When animal development is discussed, an organism and embryogenesis takes center stage. Germline initiation is an appendant event during embryogenesis, while meiosis and gametogenesis are only two events of germline differentiation.

A similar situation occurs in our understanding of plant development. The focus is mainly on the morphogenesis of multicellular structures. However, if we take the standpoint that the SRC emerged in protists, an obvious inference is that multicellular structures all emerged within the framework of the SRC. Is this possible? If we accept the argument that cell growth and division are coupled with optimal cell volume Buchanan et al.

In unicellular protists, cells in these two intervals would be mainly freely living in a population. However, under certain conditions, for example nutritional shortages, the free-living cells might be aggregated or organized, such as in Dictyostelium Dormann et al. What would result if the conditional aggregations or organizations were genetically fixed?

Multicellular organisms! If this speculation is valid, what is the relationship between the core cells involved in the SRC, such as the zygote, meiotic cells and gametes, and those involved in multicellular structure morphogenesis?

Different from simply maintaining cell volume through cell division, organized multicellular structures ultimately facilitate energy acquisition and environmental adaptation. All elaborated sex differentiation and mating behaviors can be viewed as nothing more than modifications of multicellular structures evolved afterward to facilitate meeting, recognition, and proper fusion of the two gametes.

From this comparison, we observe three different strategies of morphogenesis. In animals, the multicellular structures soma are interpolated at the first interval between zygote and meiotic cells as rest of embryos in addition to germlines. In fungi, the multicellular structures are interpolated at the second interval between meiotically produced cells and gametogenic cells, with unknown mechanisms of cell aggregation Meskauskas et al.

In plants, multicellular structures are interpolated at both intervals, with an additive strategy Bai and Xu, Comparison of morphogenetic strategies of animals, fungi, and plants in the framework of SRC.

Figure 1: Recombination is the exchange of genetic material between homologous chromosomes. At the end of prophase I, the nuclear membrane finally begins to break down. Outside the nucleus, the spindle grows out from centrosomes on each side of the cell.

As in mitosis, the microtubules of the spindle are responsible for moving and arranging the chromosomes during division. Metaphase I. Figure 2: Near the end of metaphase I, the homologous chromosomes align on the metaphase plate. Each chromosome looks like an elongated X-shaped structure. In the pair of chromosomes at top, the chromosome at left is mostly green, but the lower region of the right chromatid is orange.

The chromosome at right is mostly orange, but the lower region of the left chromatid is green. A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair. Mitotic spindles are located at each side of the cell.

Each spindle apparatus is composed of several white lines, representing fibers, emanating from two oval-shaped structures, representing centrosomes. The fibers attach the centrosomes to the centromeres of each chromosome. Shorter fibers also emanate from the mitotic spindle but are not attached to chromosomes. At the start of metaphase I , microtubules emerge from the spindle and attach to the kinetochore near the centromere of each chromosome. In particular, microtubules from one side of the spindle attach to one of the chromosomes in each homologous pair, while microtubules from the other side of the spindle attach to the other member of each pair.

With the aid of these microtubules, the chromosome pairs then line up along the equator of the cell, termed the metaphase plate Figure 2. Anaphase I. Figure 3: During anaphase I, the homologous chromosomes are pulled toward opposite poles of the cell.

The chromosome at right is moving toward the right-hand mitotic spindle. The chromosome is mostly orange, but the lower region of the left chromatid is green. A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair, mirroring the movements of the chromosomes above. During anaphase I, the microtubules disassemble and contract; this, in turn, separates the homologous chromosomes such that the two chromosomes in each pair are pulled toward opposite ends of the cell Figure 3.

This separation means that each of the daughter cells that results from meiosis I will have half the number of chromosomes of the original parent cell after interphase. Also, the sister chromatids in each chromosome still remain connected. As a result, each chromosome maintains its X-shaped structure.

Telophase I. Figure 4: Telophase I results in the production of two nonidentical daughter cells, each of which has half the number of chromosomes of the original parent cell. As the new chromosomes reach the spindle during telophase I , the cytoplasm organizes itself and divides in two.

There are now two cells, and each cell contains half the number of chromosomes as the parent cell. In addition, the two daughter cells are not genetically identical to each other because of the recombination that occurred during prophase I Figure 4.

At this point, the first division of meiosis is complete. The cell now rests for a bit before beginning the second meiotic division. During this period, called interkinesis , the nuclear membrane in each of the two cells reforms around the chromosomes.

In some cells, the spindle also disintegrates and the chromosomes relax although most often, the spindle remains intact. It is important to note, however, that no chromosomal duplication occurs during this stage. What happens during meiosis II? Prophase II. As prophase II begins, the chromosomes once again condense into tight structures, and the nuclear membrane disintegrates. In addition, if the spindle was disassembled during interkinesis, it reforms at this point in time. Metaphase II.

Figure 5: During metaphase II, the chromosomes align along the cell's equatorial plate. The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle.

The haploid multicellular plants are called gametophytes because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes because the organism that produces the gametes is already a haploid.

Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores.

The spores will subsequently develop into the gametophytes. Alternation of Generations : Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism.

In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes because they produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte.

Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly.

In plants such as moss, the gametophyte organism is the free-living plant, while the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living; however, the sporophyte is much larger.

In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte. Sexual reproduction takes many forms in multicellular organisms. However, at some point in each type of life cycle, meiosis produces haploid cells that will fuse with the haploid cell of another organism.

The mechanisms of variation crossover, random assortment of homologous chromosomes, and random fertilization are present in all versions of sexual reproduction. The fact that nearly every multicellular organism on earth employs sexual reproduction is strong evidence for the benefits of producing offspring with unique gene combinations, although there are other possible benefits as well. Privacy Policy. Skip to main content. Meiosis and Sexual Reproduction.

Search for:. Sexual Reproduction. Advantages and Disadvantages of Sexual Reproduction The genetic diversity of sexual reproduction, observed in most eukaryotes, is thought to give species better chances of survival. Learning Objectives Describe the benefits of sexual reproduction. Key Takeaways Key Points The variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population.

In sexual reproduction, different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes; this results in an increase of genetic diversity. On average, a sexually-reproducing population will leave more offspring than an otherwise similar asexually-reproducing population. Key Terms sexual reproduction : Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms.

There are two main processes during sexual reproduction: meiosis, involving the halving of the number of chromosomes, and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. Life Cycles of Sexually Reproducing Organisms The main categories of sexual life cycles in eukaryotic organisms are: diploid-dominant, haploid-dominant, and alternation of generations.

Learning Objectives Explain the life cycles in sexual reproduction.