scholarly journals Consequences of recombination for the evolution of the mating type locus in Chlamydomonas reinhardtii

2019 ◽  
Author(s):  
Ahmed R. Hasan ◽  
Jaspreet K. Duggal ◽  
Rob W. Ness
Keyword(s):  
Chlamydomonas Reinhardtii ◽  
Gene Conversion ◽  
Mating Type ◽  
Sex Chromosomes ◽  
Purifying Selection ◽  
Recombination Suppression ◽  
Type Locus ◽  
Mating Type Locus ◽  
Sexual Characteristics ◽  
Population Genetic Variation

SummaryRationaleRecombination suppression in sex chromosomes and mating type loci can lead to degeneration due to reduced selection efficacy and Muller’s ratchet effects. However, genetic exchange in the form of non-crossover gene conversions may still take place within crossover-suppressed regions. Recent work has found evidence that gene conversion may explain the low levels of allelic differentiation in the dimorphic mating type locus (MT) of the isogamous alga Chlamydomonas reinhardtii. However, no one has tested whether gene conversion is sufficient to avoid the degeneration of functional sequence within MT.MethodsHere, we calculate levels of linkage disequilibrium (LD) across MT as a proxy for recombination rate and investigate its relationship to patterns of population genetic variation and the efficacy of selection in the region.ResultsWe find that levels of LD predict selection efficacy across MT, and that purifying selection is stronger in shared genes than MT-limited genes to the point of being equivalent to that of autosomal genes.ConclusionsWe argue that isogamous systems without secondary sexual characteristics exhibit reduced selective pressure to differentiate sex chromosomes, and that recombination via gene conversion plays an important role in both reducing differentiation and preventing degeneration of crossover suppressed mating type loci.

scholarly journals The iso1 gene of Chlamydomonas is involved in sex determination.

1995 ◽  
Vol 6 (1) ◽  
pp. 87-95 ◽  
Author(s):  
A M Campbell ◽  
H J Rayala ◽  
U W Goodenough
Keyword(s):  
Sex Determination ◽  
Chlamydomonas Reinhardtii ◽  
Mating Type ◽  
Dna Sequences ◽  
Sexual Differentiation ◽  
Type Locus ◽  
Mating Structure ◽  
Mating Type Locus

Sexual differentiation in the heterothallic alga Chlamydomonas reinhardtii is controlled by two mating-type loci, mt+ and mt-, which behave as a pair of alleles but contain different DNA sequences. A mutation in the mt minus-linked imp11 gene has been shown previously to convert a minus gamete into a pseudo-plus gamete that expresses all the plus gametic traits except the few encoded by the mt+ locus. Here we describe the iso1 mutation which is unlinked to the mt- locus but is expressed only in minus gametes (sex-limited expression). A population of minus gametes carrying the iso1 mutation behaves as a mixture of minus and pseudo-plus gametes: the gametes isoagglutinate but they do not fuse to form zygotes. Further analysis reveals that individual gametes express either plus or minus traits: a given cell displays one type of agglutinin (flagellar glycoprotein used for sexual adhesion) and one type of mating structure. The iso1 mutation identifies a gene unlinked to the mating-type locus that is involved in sex determination and the repression of plus-specific genes.

scholarly journals GENE CONVERSION OF THE MATING-TYPE LOCUS IN SACCHAROMYCES CEREVISIAE

Genetics ◽  
1979 ◽  
Vol 92 (3) ◽  
pp. 777-782
Author(s):  
Amar J S Klar ◽  
Seymour Fogel ◽  
Karin Lusnak
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Gene Conversion ◽  
Mating Type ◽  
Tetrad Analysis ◽  
Type Locus ◽  
Mating Type Locus ◽  
Gene Conversions

ABSTRACT Tetrad analysis of MAT  a/MATα diploids of Saccharomyces cerevisiae generally yields 2 MATa:2MATα meiotic products. About 1 to 1.8% of the tetrads yield aberrant segregations for this marker. Described here are experiments that determine whether the aberrant meiotic segregations at the mating-type locus are ascribable to gene conversions or to MAT switches, that is, to mating-type interconversions. Diploid strains incapable of switching MAT  a to MATα, or the converse, nevertheless display changes of MAT  a to MATα, or the reverse. These events must be attributed to gene conversion. Further, we suggest that MAT  a and MATα alleles may represent nonhomologous sequences of DNA since they fail to display postmeiotic segregations.

scholarly journals Size variation of the non-recombining region on the mating-type chromosomes in the fungal Podospora anserina species complex

2021 ◽  
Author(s):  
Fanny E Hartmann ◽  
S Lorena Ament-Velásquez ◽  
Aaron A Vogan ◽  
Valérie Gautier ◽  
Stephanie Le Prieur ◽  
...  
Keyword(s):  
Mating Type ◽  
Sex Chromosomes ◽  
Chromosomal Rearrangements ◽  
Size Variation ◽  
Sequence Divergence ◽  
Podospora Anserina ◽  
Strong Linkage Disequilibrium ◽  
Mating Types ◽  
Type Locus ◽  
Mating Type Locus

Abstract Sex chromosomes often carry large non-recombining regions that can extend progressively over time, generating evolutionary strata of sequence divergence. However, some sex chromosomes display an incomplete suppression of recombination. Large genomic regions without recombination and evolutionary strata have also been documented around fungal mating-type loci, but have been studied in only a few fungal systems. In the model fungus Podospora anserina (Ascomycota, Sordariomycetes), the reference S strain lacks recombination across a 0.8 Mb region around the mating-type locus. The lack of recombination in this region ensures that nuclei of opposite mating types are packaged into a single ascospore (pseudo-homothallic lifecycle). We found evidence for a lack of recombination around the mating-type locus in the genomes of 10 P. anserina strains and six closely related pseudo-homothallic Podospora species. Importantly, the size of the non-recombining region differed between strains and species, as indicated by the heterozygosity levels around the mating-type locus and experimental selfing. The non-recombining region is probably labile and polymorphic, differing in size and precise location within and between species, resulting in occasional, but infrequent, recombination at a given base pair. This view is also supported by the low divergence between mating types, and the lack of strong linkage disequilibrium, chromosomal rearrangements, trans-specific polymorphism and genomic degeneration. We found a pattern suggestive of evolutionary strata in P. pseudocomata. The observed heterozygosity levels indicate low but non-null outcrossing rates in nature in these pseudo-homothallic fungi. This study adds to our understanding of mating-type chromosome evolution and its relationship to mating systems.

scholarly journals Consequences of recombination for the evolution of the mating type locus in Chlamydomonas reinhardtii

2019 ◽  
Vol 224 (3) ◽  
pp. 1339-1348 ◽  
Author(s):  
Ahmed R. Hasan ◽  
Jaspreet K. Duggal ◽  
Rob W. Ness
Keyword(s):  
Chlamydomonas Reinhardtii ◽  
Mating Type ◽  
Type Locus ◽  
Mating Type Locus

Localization of the nic-7, ac-29 and thi-10 genes within the mating-type locus of Chlamydomonas reinhardtii.

Genetics ◽  
1995 ◽  
Vol 141 (2) ◽  
pp. 543-549 ◽  
Author(s):  
P J Ferris
Keyword(s):  
Chlamydomonas Reinhardtii ◽  
Mating Type ◽  
Selectable Markers ◽  
Type Locus ◽  
Physical Location ◽  
Mating Type Locus ◽  
Tight Linkage ◽  
Left And Right ◽  
R Domain

Abstract The tight linkage observed between the mating-type (mt) locus of Chlamydomonas reinhardtii and three auxotrophic mutations--nic-7 (nicotinamide-requiring), ac-29 (acetate-requiring), and thi-10 (thiamine-requiring)--has led to the hypothesis that recombination is suppressed in the mt region. The physical location of these three genes has been established by transformation with sets of cloned DNA from the mt region. They lie to the left and right of the highly rearranged (R) domain of the mt locus, which has been proposed to be responsible for teh recombinational suppression in the region. The cloned nic-7+ and thi-10+ genes will be useful as selectable markers for cotransformation markers for cotransformation experiments.

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