A survey of DNA content variation and its molecular basis in Helianthus annuus

Abstract

Intraspecific differences in DNA content are frequently due to altered copy number of families of repeated, nongenic sequences for which functional roles have not been determined. There have been several reports of variation in nuclear DNA content within Helianthus annuus (2C=3.57 pg to 2C=9.90 pg). One group has attributed this in part to the loss of chromosomes during somatic development (aneusomaty). The objectives of this study were to: 1. develop a highly sensitive flow cytometric method to measure plant nuclear DNA content, and to quantify the extent of nuclear DNA content variation found in the cultivated sunflower, H. annuus; 2. resolve contradictions in measured DNA content of H. annuus reported in the literature, using this system; 3. determine if chromosome number varies in H. annuus somatic tissues (using root tip tissue), as has been reported; and 4. identify specific DNA sequences responsible for DNA content variation in H. annuus. In this study nuclear DNA variation was quantified in 14 lines of H. annuus using laser flow cytometry and, for some lines, Feulgen microspectrophotometry as well. DNA values varied at least 32% between line means. One line showed intralinear variation in DNA content of 19.08%, and leaf-to-leaf variation as high as 44.65% was found in two. No evidence of somatic chromosome number variation was observed. Copy number of seven repeated sequences (including ribosomal and histone probes) was determined in six H. annuus lines with varying DNA content; the copy number of some of these sequences were found to be correlated with DNA content variation.