Conservation of genetic linkage in nonisogenic isolates of Candida albicans

Abstract

A number of laboratories are now engaged in the genetic analysis of Candida albicans. This diploid yeast, the major fungal pathogen of humans, is imperfect. Parasexual techniques have been devised for complementation and recombination analysis in this organism. This paper attempts to address the question of the extent to which nonisogenic strains of C. albicans have conserved a common genetic map. This analysis is a prerequisite for the integration of work done in different laboratories and may also provide useful information on the taxonomy of the genus Candida. The paper also reports the analysis of an interspecific hybrid between C. albicans and Candida stellatoidea. The method employed in these studies was the analysis of the mitotic recombination relationships of a group of linked genes and their centromere. Strains carrying linked auxotrophic mutations were fused with isogenic and nonisogenic complementary strains to form tetraploids. The mitotic recombination analyses of these tetraploids suggest that in the isolates studied the genetic map is conserved. A comparison of tetraploid and diploid mitotic recombination analyses is also presented. Candida albicans, a common causative agent of mycotic infection, is generally considered to exist naturally only in the asexual state (2). Recently, parasexual protocols have been reported for this organism (5, 7, 8, 10). The protocols for parasexual analysis all employ enzymic digestion to produce protoplasts, followed by the induction of protoplast fusion by polyethylene glycol. Fusion of complementing auxotrophs generates prototrophic fusion products which can be selected on appropriate media. Such fusion products are either multinucleate heterokaryons or, more rarely, uninucleate cells which have undergone karyogamy. Uninucleate prototrophic fusion products are spontaneously unstable, giving rise with a low frequency to auxotrophic derivatives (8). The mitotic instability offusion products can be increased by exposure to UV irradiation (8). It has been suggested that the predominant form of mitotic instability is mitotic crossing-over (7, 10). The ploidy of C. albicans remains a matter of debate, some reports supporting haploidy (10) and others diploidy (1, 4, 7, 9, 11, 12). It has been suggested that protoplast fusion of naturally diploid cells gives rise to euploid tetraploids (8), but it may also give rise to aneuploid products (10). The first report of genetic linkage in C. albicans was provided by Sarachek et al. (10) with derivatives of C. albicans WC. The report indicated that the arg allele carried by WC-5-4 and

Completion of a parasexual cycle in Candida albicans by induced chromosome loss in tetraploid strains

“…In another study, tetraploid cells heterozygous for a partially dominant selectable marker were constructed by protoplast fusion. Following selection for drug resistance, 70% of the colonies picked were found to have lost chromosomes, with about half of these cells close to a diploid level (Poulter and Hanrahan, 1983). These results indicate that heat shock or drug selection can induce mitotic instability and chromosome loss, with diploid cells a significant product.…”

MATING IN CANDIDA ALBICANS AND THE SEARCH FOR A SEXUAL CYCLE

“…Tetraploid cells that had undergone chromosome loss demonstrated a range of ploidy, although in the study by Bennett & Johnson (2), approximately one third of the cells were diploid in DNA content, as determined by fluorescence-activated cell sorting and by analysis of genetic markers. Previous experiments had also showed instability of C. albicans tetraploids; in these cases the tetraploids were formed by fusion of diploid spheroplasts or protoplasts (24,54). Subsequent treatment with heat shock or drug selection induced mitotic instability and chromosome loss, producing a number of diploid progeny.…”

Genetics of Candida albicans.

“…The major use of spheroplast fusion to establish linkage and gene order has come from the work of Poulter and co-workers (36,(86)(87)(88)(89). Using mitotic recombination and heat shock, they have defined three linkage groups.…”