Genetics of sex determination in the social amoebae

Abstract

The social amoebae possess a sexual cycle that involves transient mutlicellularity: first a zygote attracts surrounding haploid amoebae to form a walled aggregate around it, and then cannibalizes these peripheral cells, eventually forming a dormant single-celled macrocyst. Self-fertile homothallic isolates occur as well as breeding groups of self-infertile heterothallic cells, which commonly have more than two mating types. The mating-type locus of the widely studied model organism Dictyostelium discoideum, which has three mating types, has recently been identified. Two of the three mating types are determined by single putative regulatory genes bearing no mutual similarity, while the third is specified by homologues of both of these genes. This is the first sexdetermining locus of an Amoebozoan to be described and, since none of the key regulators show homology to known proteins, may be a first glimpse of a novel mode of regulation used in these organisms. The sexual cycle of dictyostelids has been relatively neglected, but continues to yield much interesting biology as well as having the potential to add to the genetic tools available for the study of these organisms.

Origins of Eukaryotic Sexual Reproduction

“…In most modern eukaryotes, this handicap has been assuaged by the evolution of countless systems for identifying the opposite type via pheromones and mating behaviors, enhancing the efficiency of finding a mate, but options are still limited to half the population. In some lineages, the odds have been improved by the presence of multiple mating types-three or more in Dictyostelium (Bloomfield et al 2010(Bloomfield et al , 2011, seven in Tetrahymena (Phadke et al 2012;Cervantes et al 2013;Umen 2013), 13 in Physarum polycephalum (Collins and Tang 1977;Clark and Haskins 2010), and often thousands in the basidiomycetes (Raper 1966)-and in these cases more encounters are sexually fertile (.98% in some basidiomycetes). But more outcrossing is not always better, and in basidiomycete fungi there are multiple independent transitions from the tetrapolar outcrossing species with thousands of sexes back to bipolar species with just two mating types, where it has been suggested that different environments favor outcrossing versus inbreeding sexual cycles ).…”

Signalling and sex in the social amoebozoans

“…The research of Bloomfield et al (2010) and Bloomfield (2011) has yielded some major insights into the regulation of sexual development in the social amoebozoans. The point now is to reconcile this with past work which can more effectively guide future research in the field.…”

Sex and macrocyst formation in Dictyostelium

“…Dictyostelids and their close rela-tives the myxogastrids are among the exceptions to this rule, along with certain ciliates and basidiomycete fungi (Hurst and Hamilton, 1992). Relatedly, certain dictyostelid and myxogastrid isolates (and many fungi) are homothallic, so that genetically identical gametes are sexually compatible, unlike heterothallic isolates in which only gametes of different mating types can mate (Bloomfield, 2011). The main evolutionary driver for increased number of mating types (and homothallism), as mentioned above, is thought to be selection to maximise the probability of compatibility with potential sexual partners.…”

“…Importantly, the dictyostelid sexual cycle displays features that are unique or rare among eukaryotic sexual biology, which presumably reflects in large part from the biological imperatives imposed by these organisms' unusual life history. First, each species (at least in the genus Dictyostelium as recently redefined (Sheikh et al, 2018)) often has more than two mating types, perhaps resulting from selection to increase the probability of sexual compatibility between conspecific cells that encounter each other at the initiation of sex (Bloomfield, 2011). Second, there appears to be no block to multiple fusion of gametes during zygosis: tens or hundreds of gametes can fuse together over a period of several hours before measurable nuclear fusion occurs, allowing mixture of cytoplasms and exchange of mitochondria (Bloomfield et al, 2019;Ishida et al, 2005;Saga et al, 1983).…”