8.1 The process of sexual reproduction

8.1 The process of sexual reproduction

The core features of sexual reproduction are conserved throughout the eukaryotic tree of life, and are therefore thought to have evolved once and to have been a character of the Eukaryote Last Common Ancestor (ELCA; see Fig. 2.11 and Moore, 2013 [pp. 174 et seq.]). It follows that sexual reproduction in present day organisms displays a mixture of features that are ancient and ancestral, together with others that have arisen during the subsequent evolution of that organism. For example, sexual reproduction in the great majority of eukaryotes alive today involves two contrasting sexes or mating types, so this may be considered an ancestral feature. Yet amongst the fungi there are species that indulge in unisexual reproduction, where a single mating type can undergo self-fertile (or homothallic) reproduction on its own, either with itself or with other members of the population of the same mating type. Unisexual reproduction occurs in several different lineages and may therefore be interpreted as a derived feature that has arisen independently in those different lineages. On the other hand, the incredible variety of different types of sex (or mating type) determining mechanisms that can be observed in animals, plants, protists, and fungi of the present day may suggest that specification of sex (or mating type) is not the ancestral feature but is a derived trait; and if this is the case, then the original form of sexual reproduction may have been unisexual, onto which sexes were imposed independently in the different lineages as they evolved.

We do not know what ELCA was like but the current belief is that our last common ancestor was a unicellular, aquatic, motile creature with one or two flagella (and really rather like the sort of thing that eventually became a chytrid fungus; see Moore, 2013). Of course, as the eukaryote’s last common ancestor, ELCA certainly had a membrane-bound nucleus, mitochondria, secretory apparatus, the ability to regulate gene expression with interfering RNAs (RNAi), and the ability to reproduce both asexually and sexually (mitosis and meiosis are conserved processes throughout eukaryotes). So, when we settle down with a glass of wine and think about sex … first evolving, we think it must have occurred first in an aqueous environment (probably in some primitive biofilm) and that it involved swimming cells, and changes in ploidy that needed a reduction division (meiosis) to correct. Although cell-cell and nucleus-nucleus fusion are prominent in sexual reproduction today, there may have been a time in the distant past when internal replication cycles (endoreplication) caused the change in ploidy that needed to be corrected by a reduction division (meiosis) during ancestral attempts at sexual reproduction. This view predicts that cell-cell fusion may be ancient, but perhaps not as ancient as other features of sexual reproduction (Heitman, 2015).

It is the potential benefits of sexual reproduction that have given it the competitive edge and caused sex to be so pervasive in the eukaryotic tree of life. These benefits include that it provides a means to purge the genome of (vegetatively accumulated) deleterious mutations; and a means to shuffle the genome by means of chromosome reassortment and recombination to give rise to different gene arrangements among the meiotic progeny. Sex may also enable organisms to compete with pathogens, some of which may be internal, like transposons (see discussion and references in (Heitman, 2015). The potential benefits of sex must be balanced against the costs of sexual reproduction: that only 50% of a parental genome is transmitted to any given progeny, the time and energy required to locate mates, and the reassortment of already adapted gene arrangements.

The core features of sexual reproduction are conserved in organisms as diverse as the model budding yeast Saccharomyces cerevisiae and humans, despite at least a billion years of evolution separating us from our last shared ancestor. These conserved features include:

  • Regular changes in ploidy, from haploid to diploid to haploid, or from diploid to haploid to diploid.
  • The process of meiosis that enables meiotic recombination and halves the ploidy of the genome.
  • Cell-cell fusion (syngamy) between mating partners or their gametes.

The conservation of these core features of sexual reproduction across this enormous evolutionary time is what indicates the antiquity of the process (Billiard et al., 2012; Heitman, 2015). Most fungi can undergo both asexual reproduction and sexual reproduction. The evolutionary persistence of eukaryotes that rely on asexual reproduction alone is exceptional (see Section 7.1, above). Examples include rotifers, glomeromycotan fungi, some arthropods and some plants; but even these exceptional examples of asexuality are uncertain because molecular analyses show that genes required for the sexual cycle are maintained in their genomes, so it may simply be that their cryptic sexual stages have not yet been observed (Billiard et al., 2012).

Eventually, for the majority of fungi of the present day, karyogamy and meiosis take place and the nuclear products of meiosis are packaged into sexual spores. In many fungi sexual spores have thickened walls; that is, they are resistant spores that are often dormant, and formed in relatively small numbers. In some cases the whole gametangium (the zygospores of zygomycetes would be a typical example [CLICK HERE for a reminder of the images) develops into a resistant structure, in other cases the sexual spores (particularly ascospores) are resistant and have a period of obligate dormancy. However, in Basidiomycota, basidiospores are produced in large numbers and are dispersal spores, not dormant spores [CLICK HERE for a reminder of the appropriate section in Chapter 3].

As befits its use in traditional taxonomy there are numerous variations in sexual reproduction in fungi. The first of these variables is the presence or absence of incompatibility systems. For example, in the zygomycetes, Mucor mucedo is heterothallic (self-sterile), but its relative Rhizopus sexualis is homothallic (self-fertile).

There is then the matter of the morphology of the hyphal structures involved in the various stages and the manner in which the processes are carried out. For example, gametangia are morphologically alike in the true fungus (zygomycete) Mucor mucedo, but morphologically different in some of the Oomycota (kingdom Straminipila) like Pythium, which is an important pathogen causing damping-off of seedlings.

Similarly, the duration of the various stages of sexual reproduction may vary and some may be prolonged, for example prolonged karyogamy in diploid yeasts and, as indicated above, prolonged plasmogamy in the dikaryotic heterokaryon of Basidiomycota.

Hormones are probably involved in regulating sexual reproduction in most organisms, and fungi are no exception. Unfortunately, only a few of the active chemicals have been isolated from fungi; however, all of the major chemical classes of hormones identified in animals and plants are also known in fungi [CLICK HERE to view our Resources Box on pheromones in fungi]:

  • sterols in the Oomycete Achlya bisexualis, female mycelium produces antheridiol, male produces oogoniol [CLICK HERE to see Section 3.10];
  • the sesquiterpene hormone sirenin produced by female zoogametes of Allomyces macrogynus to attract male zoogametes [CLICK HERE to see Section 3,4];
  • chemotropism to volatile precursors in the trisporic acid pathway that attracts heterothallic (self-sterile + and -) zygophores of Mucor mucedo to one another. On their own, neither strain can produce trisporic acid, but they ‘converse’, by exchanging a volatile precursor and collaborate in its biosynthesis (Lee & Heitman, 2014); [CLICK HERE to see our Resources Box on pheromones in fungi];
  • peptide pheromones involved in yeast mating (see Section 8.2 below; CLICK HERE to see it now);
  • mating type pheromones of filamentous Ascomycota and Basidiomycota that are part of a G-protein signalling pathway (see Section 8.5 below).

As in other eukaryotes, fungi have tightly regulated mechanisms that determine which haploid cells can fuse at syngamy; but fungi display a variety of life cycles and have additional possibilities for syngamy as compared to plants and animals. Heterothallic fungi require two compatible partners to produce sexual spores, whereas in homothallic fungi a single organism is capable of sexual reproduction. In fungi considered to be heterothallic, haploid selfing is prevented because they have a mating system that ensures syngamy can only occur between haploid cells carrying different mating type genes (Billiard et al., 2011).

Mating systems (also called breeding systems) rely on nuclear genes that control progress towards meiosis in the heterokaryon established between vegetatively-compatible mycelia. Basic analysis of such systems depends on making experimental confrontations between mycelia and scoring whether or not the sexual stage is completed. Such experiments test for the phenotype of sexual reproduction, and the pattern of its occurrence and its inheritance allow deductions about the control of sexual reproduction. A mycelium that possesses genes that prevent mating between mycelia that are genetically identical will be self-sterile; since it ensures that different mycelia must come together for a successful mating to occur and this is why such a system is called heterothallism.

The genes that determine mating in fungi reside at one or two specialised chromosomal regions known as the mating type loci (MAT). These genes determine haploid cell identity (conferring the cell’s ‘mating type’); they enable compatible mating partners to attract each other, and they prepare cells for sexual reproduction after the fertilisation event (which is usually hyphal cell fusion). Some of these genes (and probably the ancestral ones) encode proteins belonging to a class of transcription factor known as the High Mobility Group (HMG) proteins. HMG proteins (named according to their mobility in polyacrylamide electrophoresis gels) are the largest class of nonhistone proteins found in the nucleus and are often found in association with regions of active transcription in chromatin (Casselton, 2008).

The transcription factors are responsible for coordinating haploid cell type specificity (‘mating type’) when working alone, and the fate of the diploid (yeast zygote) or dikaryon (filamentous fungi) when two compatible MAT loci work together as a partnership after syngamy. They can work together because they each produce homeodomain proteins that must form a dimer to function. Homeodomain proteins are evolutionarily conserved proteins which are present in the entire eukaryote kingdom, and most act as transcription factors and bind to DNA to control the activity of specific sets of genes involved in a very diverse range of functions. They bind to DNA in the promoter region of their target genes as complexes with other transcription factors; the specificity (in terms of which genes to regulate) derives from the specific collection of transcription factors that assemble at the promoter to make up the complex (Bobola & Merabet, 2017). Compatible MAT genes produce different homeodomain proteins (generally called HD1 and HD2) that form a heterodimer, that is, a dimer of HD1+HD2, which is necessary for the mating to be compatible. Neither HD1+HD1 nor HD2+HD2 form functional dimers, though both are perfectly good haploid cells.

Many heterothallic fungi, indeed all known heterothallic Ascomycota, have only two mating types specified by a single locus with different ‘alleles’: Neurospora crassa, budding yeast Saccharomyces cerevisiae, and the (basidiomycete) grass rust Puccinia graminis are examples. In such cases the mating type of a culture depends on which ‘allele’ it has at the single mating type locus (involvement of one mating type locus gives rise to the alternative name of unifactorial incompatibility): successful mating only taking place between cells or mycelia that have different ‘alleles’ at the mating type locus. Of course, the diploid nucleus that results is heterozygous for the mating type factor, and meiosis produces equal numbers of progeny of each of the two mating types (hence yet another alternative name, bipolar heterothallism).

We put the word allele into quotes in the last few sentences because, although it is not evident from classical genetic analysis, one of the first things that molecular analysis revealed about the mating type factors is that the different forms of the mating type locus do not share the amount of DNA sequence homology you would expect of alleles. Their ‘alleles’ can be very different indeed, in some cases differing in length by thousands of base pairs. For this reason they have been called idiomorphs rather than alleles. Idiomorphic structure (not allelism) is common to all fungal mating type genes that are known.

In homothallic (self-fertile) fungi sexual reproduction can occur between genetically identical hyphae, but mating type factors may still be involved. Primary homothallism occurs in species completely lacking heterothallism, but secondary homothallism occurs in species that have an underlying heterothallism that is bypassed by the inclusion of two haploid nuclei of opposite mating types and from a single meiosis when spores are made; it’s also called ‘pseudo-homothallism’ or automixis (Billiard et al., 2012).

Neurospora tetrasperma, Coprinopsis bisporus and Agaricus bisporus are good examples. In these cases, there are more post-meiotic nuclei than spores, so the spores become binucleate and heterozygous for mating type factors. Spore germination gives rise to heterokaryotic mycelia that are, consequently, able to complete the sexual cycle alone, that is they act like homothallic mycelia because they are heterokaryons right from the start. The presence of nuclei of the two mating types within dispersal spores ensures that the progeny spore will be able to mate with any haploid it encounters, and pseudo-homothallism will also allow mating between sibling spores (that is spores from the same meiosis) when no other mating is available.

The terms ‘homothallism’ and ‘heterothallism’ are also used to describe reproductive phenomena in oomycetes, though the genetic basis and the evolutionary consequences are strikingly different from true fungi. In oomycetes, which are phylogenetically closer to brown algae and diatoms than to fungi, ‘heterothallism’ and ‘homothallism’ are used to describe how sexual reproduction is started: heterothallic oomycetes cannot undergo gamete production and sexual reproduction unless an individual of the opposite mating type is present, while homothallic oomycetes can (Judelson, 2007). Once gametes are produced, diploid selfing and outcrossing are possible in both homothallic and heterothallic oomycetes. The terms ‘homothallism’ and ‘heterothallism’, which are often used interchangeably with the terms ‘selfing’ and ‘outcrossing’ in both oomycetes and fungi, correspond to different phenomena in these different organisms. Confusion over different usages of these terms, especially ‘selfing’, stems from the fact that ascomycetes have a long-lasting haploid mycelial stage, so that the ‘individual’ which may be indulging in ‘self-fertilisation (selfing)’ is the haploid mycelium which may be heterokaryotic (see discussion in Billiard et al., 2012).

The yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe exemplify a different process. Most strains are heterothallic with two mating types (see below), but in some strains mating occurs between progeny of a single haploid ancestor; that is, the culture appears to be ‘homothallic’. This apparent homothallism results from a recombinational process that allows a switch, in a few cells in the population, from one mating type to the other (see Section 8.3 below for details [CLICK HERE to see it now]) so that the (still heterothallic) haploid clone comes to contain cells of a different mating type.

There is one last general point to make before we describe some of the details. This is that the mating type factors were discovered phenotypically and genetically long before their molecular basis was revealed. Consequently, they were named, in some cases up to a hundred years ago, in ways that were appropriate then, but may not seem appropriate now. Bipolar mating types may be +/- (plus and minus, now more often called P and M), or A and a, or a and α (‘a’ and ‘alpha’). Where mating type is determined by two mating type factors (tetrapolar heterothallism) they are called A and B, one encodes the homeodomain proteins, and the other encodes pheromones and pheromone receptors; but which locus encodes which gene products varies with the species, because the genes were named historically as they were identified without knowledge of their molecular function.

Updated July, 2018