A quantitative assay to measure chromosome stability in Schizosaccharomyces pombe

Summary The fidelity of mitotic chromosome transmission in Schizosaccharomyces pombe was estimated quantitatively by using cycloheximide resistance as a means to select cells that had undergone chromosome loss or nondisjunction. We aimed to investigate the connection between recombination and mitotic chromosome stability. A number of mutants defective in mitotic recombination such as cdc17-L16, rec59-72, and rec50-25 were tested and in these an approximately ten fold elevation of mitotic haploidization rate was found compared with controls. Our data suggest that recombination is important in controlling the maintenance of chromosomes during mitosis.     

Isolation and Characterization of Schizosaccharomyces Pombe Mutants Affected in Mitotic Recombination

A haploid Schizosaccharomyces pombe strain carrying a heteroallelic duplication of the ade6 gene was used to isolate mitotic recombination-deficient mutants. Recombination between the different copies of the ade6 gene can lead to Ade+ segregants. These are observed as growing papillae when colonies of a suitable size are replicated onto selective medium. We isolated mutants which show an altered papillation phenotype. With two exceptions, they exhibit a decrease in the frequency of mitotic recombination between the heteroalleles of the duplication. The two other mutants display a hyper-recombination phenotype. The 12 mutations were allocated to at least nine distinct loci by recombination tests. Of the eight rec mutants analyzed further, six were also affected in mitotic intergenic recombination in the intervals cen2-mat or cen3-arg 1. No effect on mitotic intragenic recombination was observed. These data suggest that mitotic gene conversion and crossing over can be separated mutationally. Meiotic recombination occurs at the wild-type frequency in all mutants investigated.     

Schistocephalus solidus: a molecular test of premature gamete exchange for fertilization in the intermediate host Gasterosteus aculeatus

We tested the hypothesis that the cestode Schistocephalus solidus is capable of premature gamete exchange as a plerocercoid in the last intermediate stickleback host. The existence of such a reproductive mode is suggested by the highly advanced gonadal development in the plerocercoid and the large fitness gain of outcrossing. In addition, eggs from selfing cestodes have a higher hatching rate when the cestode originated from a doubly infected stickleback than when it came from a singly infected fish. We hatched eggs from 10 singly breeding cestodes that originated from doubly infected sticklebacks with the prediction that some should be outcrossed and share alleles with both the breeding cestode and the second cestode in the coinfection if the hypothesis is correct. However, all of the 430 tested larvae matched only the alleles of the breeding cestode. It is therefore very unlikely that S. solidus engaged in gamete exchange in its fish host. We suggest an alternative hypothesis to explain the higher hatching rate of eggs produced by cestodes from doubly infected fish as compared to those from single infections.     

Size-dependent discrimination of mating partners in the simultaneous hermaphroditic cestode Schistocephalus solidus

The cestode Schistocephalus solidus is a simultaneous hermaphroditethat reproduces in the gut of birds, or for this study in an in vitro system that simulates the gut of the bird. Like manyother helminth parasites, S. solidus can reproduce by self-and cross-fertilization. Hermaphrodites are expected to matenot primarily to get their own eggs fertilized, but ratherto get the opportunity to fertilize a partner's eggs. BecauseS. solidus has a size-dependent sex allocation (i.e., largerworms are more biased toward female allocation and producemore egg mass), we expect larger individuals to be attractivemating partners for smaller ones. However, this may be a one-directionalpreference, as smaller individuals may not be attractive tolarger ones. We tested this experimentally by studying thereaction of focal worms of different sizes to a compartmentcontaining a potential mating partner that was either smalleror larger than the focal worm. The focal worms were, on average,closer to the compartment containing the stimulus than to anempty control compartment. Moreover, they indeed showed a preferencefor larger stimulus worms than for smaller ones. They eventended to avoid being close to stimulus worms of very smallsize compared to themselves. This may reveal a general preferencefor cross-fertilization over selfing, but it also indicatesthat all the genetic benefits from outcrossing do not necessarilyoutweigh the costs of mating with a relatively small individualand that the worms may take this into account in their reproductivedecisions.