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Nucleolar organizing chromosomes ofRicinus.

H S Paris1, O Shifriss, G Jelenkovic

  • 1Department of Horticulture and Forestry, Cook College, Rutgers-The State University, New Brunswick, New Jersey, USA.

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Summary
This summary is machine-generated.

Structural variations in nucleolar organizing regions (NORs) of Ricinus communis chromosomes 2 and 7 were observed among nine races. These variations, primarily in heterochromatic short arms, do not correlate with plant phenotype, suggesting intragenomic evolution.

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Area of Science:

  • Plant genetics
  • Cytology
  • Evolutionary biology

Background:

  • Ricinus communis (castor bean) exhibits 2n=20 chromosomes.
  • Pachytene chromosome morphology analysis is crucial for understanding genomic organization.
  • Nucleolar organizing regions (NORs) are key sites for ribosomal RNA synthesis and nucleolus formation.

Purpose of the Study:

  • To compare pachytene chromosome morphology in nine races of Ricinus communis.
  • To investigate structural variations in nucleolar organizing regions (NORs) and their association with nucleoli.
  • To explore the evolutionary origins of NORs and their relationship with phenotypic variation.

Main Methods:

  • Light microscopy was used to examine pollen mother cells (PMCs) from nine Ricinus communis races.
  • Pachytene chromosome morphology, specifically bivalents with NORs (chromosomes 2 and 7), was analyzed.
  • The association frequency between nucleoli and NOR-bearing chromosomes, including nucleolar constrictions (NCs), was quantified.

Main Results:

  • Structural variations were identified in the short arms of chromosomes 2 and 7, which contain the NORs.
  • The frequency of nucleolus association with NOR-bearing chromosomes varied among races, influenced by the presence and location of nucleolar constrictions (NCs).
  • One race lacked a visible NC, showing 100% nucleolus association with both NOR chromosomes.

Conclusions:

  • The two non-linked NORs in Ricinus communis likely evolved through intragenomic changes, not polyploidy.
  • Ricinus communis is sensitive to structural variations outside of NOR regions.
  • Observed structural variations in nucleolar organizing chromosomes are not linked to racial phenotypic differences.