This review discusses the evolutionary and scientific implications of considering these three events as part of a single process. Viewed in this way, the SRC is revealed to be a mechanism for efficiently increasing genetic variation, facilitating adaptation to environmental challenges. It also becomes clear that, in terms of cell proliferation, it is appropriate to contrast mitosis with the entire SRC, rather than with meiosis alone.
Evolutionarily, it appears that the SRC was first established in unicellular eukaryotes and that all multicellular organisms evolved within that framework. This concept provides a new perspective into how sexual reproduction evolved, how generations should be defined, and how developmental processes of various multicellular organisms should properly be compared. Sex is always a hot topic in human society and is an intensively investigated phenomenon in biology. Studies on sex determination in plants have focused on regulation of unisexual flower development Ainsworth, ; Bai and Xu, since plant sex was defined based on unisexual flowers by Robbins and Pearsoni.
This raises Cells created by sexual reproduction question of what sex is in angiosperms with perfect flowers with both pistils and stamens or in plants without flowers.
Even further, do sex and sexual differentiation share any features in common in the plant and animal kingdoms? Cells created by sexual reproduction understand the regulatory mechanism of sex differentiation, following the concept originated from Robbins and Pearsonwe have investigated the regulation of unisexual flower development using cucumber for more than a decade.
Cucumber is monoecious, and naturally bears both male and female flowers and, rarely, even hermaphrodite flowers, on the same plant. The ratio of male and female flowers can be affected by application of phytohormones such as ethylene increasing the proportion of female flowers or GA increasing the proportion of male flowers. As hormones play key roles in mammalian sex expression, this phytohormonal regulation of the ratio of male and female flowers led to the use of cucumber a model system for research into plant sex determination starting in the s Cells created by sexual reproduction Bai and Xu, and references there in.
However, following the discovery that both male and female flowers contain initiated stamen and carpel primordia, we found that flowers become female because stamen development is inhibited early at stage 6, and that ethylene is involved in this inhibition for detailed review, see Bai and Xu, These findings indicated that analysis of regulatory mechanisms in unisexual flowers will allow us to understand only how the inappropriate organs are inhibited, not how the organs are differentiated.
Unisexual cucumber flowers are not the only example of this type of puzzle. All unisexual flowers in monoecious plants and many in dioecious plants for which the developmental mechanisms are known result from inhibition of one type of sexual organs Bai and Xu, ; Akagi et al.
We also hypothesized that before multicellular organisms emerged, a process called the sexual reproduction cycle SRC evolved based on the existing mitotic cell cycle in unicellular eukaryotes.
This SRC starts from one diploid zygote, goes through meiosis, gametogenesis, and fertilization, and ends with two diploid zygotes. This review addresses the question of whether adopting the SRC concept can facilitate our understanding of sex. We begin with the basic facts based on which the SRC was hypothesized.
We would restrict our "Cells created by sexual reproduction" in eukaryotes. Secondly, meiosis is highly conserved in almost all known eukaryotes, including animals, plants, fungi, and protists Logsdon, ; Schurko and Logsdon, Thirdly, despite the extreme diversity of morphology and recognition mechanisms of gametes, cell fusion of two haploid gametes into a new diploid zygote is conserved in all eukaryotes.
Furthermore, heterogamy is Cells created by sexual reproduction restricted to dimorphism, but rather can include multiple mating types, e. It is worth noting that such multiple mating types are mainly found in protists and fungi, but not in plant and animal "Cells created by sexual reproduction." Fifthly, regardless of the presence or absence of germlines, e. With this view, sexual reproduction is predicted to be more ancient than multicellular structures as sexual reproduction already existed in unicellular eukaryotic organisms.
There has been much debate about how sexual reproduction evolved e. Nonetheless, we can try to explore the events involved in such evolution by analyzing the benefits for which the ancient evolutionary innovations Cells created by sexual reproduction have Cells created by sexual reproduction selected.
The first indispensable event in sexual reproduction is considered to be meiosis. It is currently agreed that the most important benefit of meiosis is increasing genetic variation through recombination. However, by definition, meiosis is
Cells created by sexual reproduction by a reduction of chromosome numbers from diploid to haploid.
How, then, did meiosis emerge and become selected? It is known that haploid cells, like diploid cells, can undergo mitosis, such as in budding yeast. Considering the complexity of chromosome organization, it would be reasonable to speculate that the earliest eukaryotic cells were haploid. If that were the case, meiosis would be predicted to have evolved not only after the emergence of mitosis, but also after the emergence of diploid cells, which may have arisen from cell fusion or chromosome duplication in haploid cells.
While many organisms in the protist and fungus kingdoms live mainly in a haploid state Campbell and Reece,almost all multicellular organisms in the animal and plant kingdoms use diploid cells as their building blocks. The prevalence of diploidy in the latter kingdoms suggests that diploidy must confer some advantages.
If we naively believe that diploidy can doubly secure the genome stability of eukaryotic cells, then it follows that haploidy provides little leeway for mistakes.
From this perspective, reduction of chromosome number would not be a good reason for meiosis to be selected. Instead, meiosis must occur and be selected for other reasons. This might be analogous to playing ringtoss: Recently Ross et al. Illustration of the role of cohesins in the origin of meiosis. A Cohesins Cells created by sexual reproduction sister chromosome together during mitosis. Probably because of meiotic recombination. Although DNA transmission from cell to cell already existed in prokaryotes, meiotic recombination is considered to be the first efficient mechanism evolved for autonomously increasing genetic variation.
This begs the question of why genetic variation would be so important for a cell that meiosis conferred an advantage during evolution. Regardless of how the first Cells created by sexual reproduction arose from an RNA world or pre-cellular biosystem, afterward the cells were relatively isolated from the environment from which they emerged.
Although the advent of the cell granted the biosystem tremendous independence and the ability to proliferate itself through cell division, it created a problem of adapting to the unpredictable changes in its environment. Spontaneous DNA mutation is the original way to adapt, but with low efficiency. By contrast, meiotic recombination can generate numerous genetic variations more efficiently. Among the variations randomly generated during meiosis could be those that are adaptive to the prevailing environmental conditions and enable cell survival in a changed environment.
Therefore, increasing genetic variation for adaptation might be a primary reason for meiosis to be selected. The stress-induced meiosis observed in protists is consistent with this speculation. It is conventionally understood that the sexual reproduction starts from meiosis and ends at fertilization. However, as we speculated previously, "Cells created by sexual reproduction" is likely that eukaryotes first emerged as haploid.
If that is the case, cell fusion, rather than cell division, should be an ancient event in the emergence of diploid cells. Regardless of whether the first eukaryotic cell emerged when one prokaryotic organism engulfed another as Margulis and Sagan summarized and of how chromosome duplication originated, without diploid cells there would be no meiosis. Since fertilization is essentially a cell fusion process, we can predict that it is derived from the ancient cell fusion mechanism.
If indeed cell fusion arose prior to meiosis, once meiosis emerged in the resulting diploid cells, fertilization could be readily used as a mechanism to restore genome diploidy. Furthermore, if we think about the genetic variation randomly increased by meiosis, fertilization actually retains the variation derived from both cells that are the products of meiosis.
In addition, considering the interaction between
Cells created by sexual reproduction meiotically produced cells and their environment, Cells created by sexual reproduction between the surviving haploid cells would actually execute a selective function in maintaining variations adaptive to that environment, as cells carrying non-adaptive variations would "Cells created by sexual reproduction" not survived to participate in fertilization.
Therefore, the advantages of fertilization include not only restoration of genome diploidy, but more significantly, Cells created by sexual reproduction retention of the selected genetic variations generated through meiosis.
However, each meiotic cell would generate four types of resulting cells after random recombination. If "Cells created by sexual reproduction" 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 Cells created by sexual reproduction 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 Volvoxexpansion of a mating locus causes heterogametes to change from being in size to being dimorphic Ferris et al. What is the advantage of heterogamy that enables the Cells created by sexual reproduction 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 Cells created by sexual reproduction 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 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 evolved. However, efficiency in gamete distribution and meeting might contribute to the shift: 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: 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 Cells created by sexual reproduction green algae, mosses, and ferns in some species like Ploypodium to before meiosis in gymnosperms and angiosperms.
"Cells created by sexual reproduction" from Bai and Xuby permission of Elsevier. Similarly, if we examine mechanisms of animal sex differentiation with a broader view, there are also diversifications worth noting: 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 transand 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 Cells created by sexual reproduction 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, "Cells created by sexual reproduction" fungi.
The gametes are produced from diploid germ cells, a special cell line that only During sexual reproduction, specialized haploid cells from two individuals join.
SEXUAL REPRODUCTION: involves two parent cells; each parent gives some made for "Cells created by sexual reproduction" biology, this is a must see animation/ description of meiosis. Sexual reproduction is a kind of life cycle where generations alternate between cells with a Biologists studying evolution propose several explanations for why sexual reproduction developed and why it is maintained.
These reasons include .