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Two Multiallelic Mating Compatibility Loci Separately Regulate Zygote Formation and Zygote Differentiation in the
P J Youngman1, R W Anderson, C E Holt
1Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
Abstract:
The mating of Physarum polycephalum amoebae, the ultimate consequence of which is a "plasmodium," was recently shown to be governed by two compatibility loci, matA (or mt) and matB (Dee 1978; Youngmanet al. 1979). We present evidence that matA and matB separately regulate two discrete stages of mating: in the first stage, amoebae (which are normally haploid) fuse in pairs, with a specificity determined by matB genotype, to form diploid zygotes; subsequent differentiation of the zygotes into plasmodia is regulated by matA and is unaffected by matB. Mixtures of amoebae carrying unlike matA and matB alleles formed diploids to the extent of 10 to 15% of the cells present, and the diploids differentiated into plasmodia. When only the matB alleles differed, diploid cells still formed to a comparable (5 to 10%) extent, but rather than differentiating, these diploids remained amoebae. When strains carried the same alleles of matB, formation of diploid cells was greatly reduced: in like-matB, like-matA mixtures, none of 320 cells examined was diploid; in like-matB, unlike mat-A mixtures, differentiating diploids could be detected, but at only 10(-3) to 10(-2) the frequency of unlike-matB, unlike-matA mixtures. The nondifferentiating diploid amoebae recovered from unlike-matB, like-matA mixtures were genetically stable through extensive growth, even though they grew more slowly than haploids (10-hr vs. 8-hr doubling period), and could be crossed with both haploids and diploids. The results of such higher ploidy and mixed ploidy crosses indicate that karyogamy does not invariably accompany zygote formation and differentiation.
Insights
Physarum polycephalum mating involves two loci, matA and matB, regulating distinct stages. matB controls initial zygote formation, while matA governs subsequent plasmodium differentiation, revealing complex genetic control in amoebae.
Area of Science:
- Cell Biology
- Mycology
- Genetics
Background:
- The mating process in Physarum polycephalum, leading to plasmodium formation, is known to be controlled by two compatibility loci: matA and matB.
- Previous research established the existence and general roles of these loci in P. polycephalum mating.
- Understanding the precise genetic regulation of mating stages is crucial for comprehending its life cycle and population genetics.
Purpose of the Study:
- To investigate the distinct regulatory roles of the matA and matB compatibility loci in Physarum polycephalum mating.
- To determine how different allelic combinations at matA and matB affect zygote formation and subsequent differentiation into plasmodia.
- To analyze the genetic stability and crossing capabilities of diploid amoebae that fail to differentiate.
Main Methods:
- Created mixtures of Physarum polycephalum amoebae with varying genotypes at the matA and matB loci.
- Quantified the formation of diploid zygotes through cell fusion in these mixtures.
- Observed and quantified the differentiation of diploid zygotes into plasmodia.
- Assessed the genetic stability and cross-fertility of non-differentiating diploid amoebae.
Main Results:
- matB genotype specifically determines the fusion of haploid amoebae to form diploid zygotes.
- matA regulates the differentiation of these diploid zygotes into plasmodia, independent of matB.
- When only matB alleles differed, diploid formation occurred, but differentiation was inhibited, resulting in stable diploid amoebae.
- Reduced diploid formation and differentiation were observed when matB alleles were the same, regardless of matA alleles.
Conclusions:
- matA and matB loci control discrete, sequential stages of Physarum polycephalum mating: zygote formation (matB) and plasmodium differentiation (matA).
- Karyogamy and differentiation are not obligate consequences of zygote formation, as evidenced by stable, non-differentiating diploid amoebae.
- The genetic stability and cross-fertility of these diploid amoebae suggest a complex ploidy dynamic in P. polycephalum.
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