4.8: Origins and implications of sexual reproduction

One type of social interaction is sexual reproduction, which involves cooperative interactions between distinct organisms. While two distinct sexes (male and female) is common, it is not universal. Many unicellular eukaryotes are characterized by a number of distinct “mating types”. Typically, sexual reproduction involves the fusion of two specialized cells, known as gametes, of different mating types or sexes. Through multiple mechanisms to be considered later, the outcome of sexual reproduction leads to increased genetic diversity among offspring.

What are the common hallmarks of sexual reproduction? Let us return to the slime mold Dictyostelium as an exemplar. We have considered its asexual life cycle, but Dictyostelium also has a sexual life cycle. Under specific conditions, two amoeboid cells of different mating types will fuse together to form a single cell. The original cells are haploid (→), meaning that they each have a single copy of their genome. After fusion, the resulting cell has two copies of its genome and is referred to as diploid. This diploid cell then goes through a series of events, known collectively as meiosis, a process that results in the shuffling of genetic material and the production of four haploid cells. The critical point is that the genotypes of the haploid cells that emerge from meiosis are different from the haploid cells that originally fused together. Some organisms can spend a significant amount of time in the haploid state, while others spend most of their lives in the diploid state. You, for example, had a reasonably short haploid stage (as both an egg AND a sperm cell); your diploid stage began when these two cells fused.

The oscillation between haploid and diploid states has some interesting implications. The first is that in the diploid state, there are (generally) two copies of each gene. The different versions of a gene present in a diploid cell can be the same or different. If they are the same, the cell/organism is said to be homozygous for that gene; if they are different, it is heterozygous for that gene. Alleles can have a range of effects on phenotype, from cellular lethality to more subtle effects arising from differences in the activity, localization, stability, specific form, or amount of the gene product. These effects may be influenced by the products of other genes, leading to genetic background effects. In diploid cells the effects of one allele can be masked by the presence the other normal (wild type) allele. Such "maskable" alleles are commonly referred to as recessive. We will return to these topics later on. Where genes are actively expressed and functionally important in the haploid state, which is not always the case, the presence of a lethal allele can lead to death or dysfunction. The presence of an extended haploid phase of an organisms’ life cycle can lead to the elimination of such alleles from the population.

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