Sexual reproduction is conserved throughout each supergroup within the eukaryotic tree of life, and therefore thought to have evolved once and to have been present in the last eukaryotic common ancestor LECA. Given the antiquity of sex, there are features of sexual reproduction that are ancient and ancestral, and thus shared in diverse extant organisms.
On the other hand, the vast evolutionary distance that separates any given extant species from the LECA necessarily implies that other features of sex will be derived. While most types of sex we are familiar with involve two opposite sexes or mating types, recent studies in the fungal kingdom have revealed novel and unusual patterns of sexual reproduction, including unisexual reproduction.
Unisexual reproduction has arisen independently as a derived feature in several different lineages. That a myriad of different types of sex determination and sex determinants abound in animals, plants, protists, and Danio rerio reproduccion asexual en suggests that sex specification itself may not be ancestral and instead may be a derived trait.
If so, then the original form of sexual reproduction may have been unisexual, onto which sexes were superimposed as a later feature. In this model, unisexual reproduction is both an ancestral and a derived trait. In this review, we consider what is new and what is old about sexual reproduction from the unique vantage point of the fungal kingdom. one knows the exact nature of the LECA, but we think that this ancestor was a unicellular, aquatic, motile creature with one or two flagella.
Thus, in some respects the LECA was simple. But in other ways, it was already quite complex, with a nucleus, mitochondria, secretory apparatus, RNAi, and reproducing both asexually and sexually. Thus, when we think of where sex first evolved, it was in the water, involving swimming cells Levin and King, ; Umen and Heitman, And when we think of how sex first evolved, this involved changes in ploidy and the process of meiosis, given their conserved nature throughout eukaryotes.
And while cell-cell and nuclear-nuclear fusion play prominent roles in sexual reproduction today, there may have been an era in which endoreplication cycles followed by meiosis drove the processes of ploidy change during ancestral modes
Danio rerio reproduccion asexual en sexual reproduction.
In this view, cell-cell fusion may be ancient, but perhaps not as ancient as other features of sexual reproduction. Why sex is so pervasive is thought to result from potential benefits conferred by sexual reproduction.
These include purging the genome of deleterious mutations and shuffling the genome via independent chromosomal assortment and recombination to give rise to a diverse repertoire of meiotic progeny.
Sex may also enable organisms to keep pace with or outrun pathogens, including those both external and those internal such as transposons. There is sound experimental evidence from studies in Caenorhabditis elegans and in naturally occurring snails in New Zealand for this last hypothesis in which sex allows species to keep pace with their pathogens King et al.
However, these potential benefits of sex are pitted against well-known costs of sexual reproduction: The
Danio rerio reproduccion asexual en features of sexual reproduction are conserved in organisms as diverse as the model budding yeast Saccharomyces cerevisiae and humans, despite a billion years or more of evolution separating us from our last common shared ancestor.
These conserved features include: This ubiquity of the conserved features of sex again speaks to the antiquity of the process. Beyond the commonalities in the mechanisms of sex, there are also shared features to the modes of sexual reproduction.
This includes outbreeding between genetically divergent members of the population, but also types of inbreeding that can involve the ability of the yeast S. And in humans there are the examples of consanguineous marriages, resulting for example from cousin-cousin pairings, which lead to considerable inbreeding with the risk of exposure of recessive alleles in a homozygous configuration.
We will return to this theme of the balance between outbreeding and inbreeding. Sex in humans and many other animals is determined by the X and Y sex chromosomes, in which individuals with XX karotype are female and those with XY are male.
The two sex chromosomes are dramatically different in size, and are referred to as heteromorphic sex chromosomes. A single gene resident on the Y chromosome, SRY, is sufficient to direct male fate and transferring this single gene from the Y to the X chromosome suffices to cause sex reversal in both humans and in mice.
But in other plants and animals, there are different mechanisms of sex determination. Some species, such as the plant Papaya and the fish Medaka, have sex chromosomes in which the sex specific region is small and the two sex chromosomes are the same size, so called homomorphic sex chromosomes Kondo et al.
Chickens and other birds have a completely different type of sex chromosome, called Z and W, and in these lineages it is the heterogametic ZW pattern that specifies female and the homogametic ZZ the male Zhou et al. In some animals, including turtles and crocodiles, the temperature at which an egg hatches determines sexual identity and this is called Environmental Sex Danio rerio reproduccion asexual en ESD to distinguish it from Chromosomal Sex Determination CSD Barske and Capel, Yet other species appear to be hybrids of the two with features of both environmental and chromosomal sex determination.
Finally in some lines of the zebra fish Danio rerio sex appears to be a quantitative trait, in Danio rerio reproduccion asexual en genes on multiple different chromosomes come together in allelic combinations that favor either female or male fate Anderson et al. This quantitative sex determining system has been termed polygenic sex determination PSD.
Recent studies of wild D. To summarize, in simple terms the ways in which sex is determined are plastic Danio rerio reproduccion asexual en diverse. Relatively few fungi have large size dimorphic sex chromosomes, but there are a few well studied examples such as Neurospora tetrasperma, Podospora, and Microbotryum Ellison et al.
Most fungi have relatively smaller regions of their genome, called mating-type loci, or MAT for short, that dictate their mating type or sex Fraser and Heitman, The paradigmatic example is S. The a haploid cell type is the default, and is not actively specified Danio rerio reproduccion asexual en the MAT locus. But other fungi have much more exotic sex lives, and have a more complex mating-type determining system in which there are literally thousands and thousands and thousands of different mating types Brown and Casselton, ; Casselton,; Heitman et al.
In these species there are two loci that lie unlinked on different chromosomes that specify mating type.
These are called the A and B MAT loci, and one encodes the homeodomain factors, and the other encodes pheromones and pheromone receptors which locus encodes which genes depends on the species, because these loci were named historically as they were discovered genetically long before their molecular basis was elucidated.
In Danio rerio reproduccion asexual en species, both loci are multiallelic, and as a result there are many different mating types. Both loci must differ for productive mating and thus an isolate of A1B1 mating type can mate with an isolate of A2B2 mating type, but not with isolates that are A1B2 or A2B1. It is thought to be these differences in the frequency of outcrossing and inbreeding that provided the evolutionary pressure for transitions between bipolar and tetrapolar mating systems.
Phylogenetic reconstructions across the fungal kingdom support the conclusion
Danio rerio reproduccion asexual en bipolar mating type is an ancestral state and the tetrapolar configuration is a derived state. This includes a myriad of species throughout the Ascomycota including S. This is in marked contrast to the Basidiomycota branch of the dikarya in which a majority of species have the tetrapolar mating system configuration. But thus far, no species with the tetrapolar mating system have been found outside of the Basidiomycota.
Thus, we can root the phylogenetic tree with multiple outgroups that are all bipolar, and conclude that the tetrapolar system is a derived state, possibly with a single origin at the base of the Basidiomycota. In essence, an ancestral system with just one MAT locus encoding homeodomain factors evolved into a system with a second sex determinant on another chromosome encoding pheromones and pheromone receptors. Some aspects of this have been modeled, conceptualizing how such an event might have transpired Fraser et al.
Interestingly, there are examples of species with bipolar mating type within the Basidiomycota phylum. Are these remnants of the bipolar ancestral state, like the microwave echoes of the Big Bang that formed our universe, or rather derived from the tetrapolar configuration?
This question has been addressed in some detail in the Cryptococcus pathogenic species complex Heitman et al. All of the pathogenic species and lineages, including C. Analysis of a series of closely aligned and related species reveals that multiple outgroup species have tetrapolar mating systems. Related studies for Kwoniella mangrovensis characterized its mating type locus and clarified that it is tetrapolar Guerreiro et al.
Taken from this vantage point of multiple
Danio rerio reproduccion asexual en outgroup closely aligned species, we can conclude that the bipolar configuration of the pathogenic species complex is a derived state and has a monophyletic origin.
The longer evolutionary pathway was bipolar to tetrapolar and then a return to bipolar.