Preconjugant cell interaction inEuplotes crassus: An analysis by temporary cycloheximide treatments

Gamete signalling underlies the evolution of mating types and their number

The gametes of unicellular eukaryotes are morphologically identical, but are nonetheless divided into distinct mating types. The number of mating types varies enormously and can reach several thousand, yet most species have only two. Why do morphologically identical gametes need to be differentiated into self-incompatible mating types, and why is two the most common number of mating types? In this work, we explore a neglected hypothesis that there is a need for asymmetric signalling interactions between mating partners. Our review shows that isogamous gametes always interact asymmetrically throughout sex and argue that this asymmetry is favoured because it enhances the efficiency of the mating process. We further develop a simple mathematical model that allows us to study the evolution of the number of mating types based on the strength of signalling interactions between gametes. Novel mating types have an advantage as they are compatible with all others and rarely meet their own type. But if existing mating types coevolve to have strong mutual interactions, this restricts the spread of novel types. Similarly, coevolution is likely to drive out less attractive mating types. These countervailing forces specify the number of mating types that are evolutionarily stable.

This article is part of the themed issue ‘Weird sex: the underappreciated diversity of sexual reproduction’.

Ultrastructural aspects of the development of the cytoplasmic connection between mating cells of the ciliate Euplotes crassus

Mating pair formation in Euplotes crassus involves homopolar orientation of two cells which eventually closely appose their ventral peristomial fields in such a way that the ciliary membranelles of the left margin of one field face the right margin of the other. Scanning and transmission electron microscopic observations revealed that the two apposed fields become united by a cytoplasmic connection, which develops at a precise site immediately adjacent to the membranelles nos. 14-18 and causes tearing off of a piece of the alveolar layer underlying the plasma membrane. The cytoplasm which starts bridging the mates' bodies contains a dense population of membrane-delimited vesicles and ribosome-like granules, and is outlined by nascent plasma membrane in some areas and preexisting plasma membrane of the cell cortex in others. In freeze-fracture replicas, this plasma membrane showed clear-cut distinctive aspects in relation to its age: whereas the preexisting plasma membrane appeared completely and uniformly covered with intramembranous particles, that nascent carried only a few particles and showed numerous irregular interruptions. These interruptions were supposed to represent sites where newly synthesized material is added to the membrane from the underlying cytoplasm.

Development of sexual maturity in the ciliate Euplotes crassus: sources of variation in the timing of maturity

The life styles of ciliated protists are particularly suitable for experimental analyses of certain aspects of developmental and genetic biology. The progression from sexual immaturity to maturity to senescence represents one of the most intriguing aspects of developmental programs. The extent to which progeny clones, their subclones, and testers used in the assay result in different lengths of immaturity has been investigated in Euplotes crassus. Six subclones from each of 12 progeny clones from a cross between stocks EC1 and EC2 were tested for maturity with stocks EC3, EC4, and EC5 on every transfer. Analysis of variance was used to partition the total variation in fissions to maturity into parts due to clones, subclones, and testers and the interactions between these levels. The error, interaction of subclones and testers, corresponds to a standard deviation of only 4.1 fissions, while the within clone within tester means range from 15.2 to 46.7 fissions; all levels except testers contribute significantly to the total variation. Most of the variability is attributable to clones (66%), the next most to error (16%), the next most to interaction of clones by testers (13%), and the least to subclones (5%). An a posteriori analysis examined whether the differences among clones were due to the cytoplasm of the clone ancestor (exconjugant), its mat (mating-type) locus genotype, or the mated pair it came from. None of these characteristics was able to interpret simply the large variability among clones. These results provide evidence that the transition from immaturity to maturity is quantitative and complex rather than a jump from one well-defined state to another.