Hybridization and genome size evolution: timing and magnitude of nuclear DNA content increases in Helianthus homoploid hybrid species

Abstract

Summary• Hybridization and polyploidy can induce rapid genomic changes, including the gain or loss of DNA, but the magnitude and timing of such changes are not well understood. The homoploid hybrid system in Helianthus (three hybrid-derived species and their two parents) provides an opportunity to examine the link between hybridization and genome size changes in a replicated fashion.• Flow cytometry was used to estimate the nuclear DNA content in multiple populations of three homoploid hybrid Helianthus species ( Helianthus anomalus , Helianthus deserticola , and Helianthus paradoxus ), the parental species ( Helianthus annuus and Helianthus petiolaris ), synthetic hybrids, and natural hybrid-zone populations.• Results confirm that hybrid-derived species have 50% more nuclear DNA than the parental species. Despite multiple origins, hybrid species were largely consistent in their DNA content across populations, although H. deserticola showed significant interpopulation differences. First-and sixth-generation synthetic hybrids and hybrid-zone plants did not show an increase from parental DNA content. Firstgeneration hybrids differed in DNA content according to the maternal parent.• In summary, hybridization by itself does not lead to increased nuclear DNA content in Helianthus , and the evolutionary forces responsible for the repeated increases in DNA content seen in the hybrid-derived species remain mysterious.

Contributions of Flowering Time Genes to Sunflower Domestication and Improvement

“…The sunflower genome is 3.5 Gb large (Baack et al 2005), and based on a map length of 1400 cM , there are 2.5 Mb/cM. HaFT2 and HaFT3 are indeed that closely linked, although HaFT2 is not present in the 10 and 100 kb of BAC sequence to either side of HaFT3.…”

A genomic view of introgression and hybrid speciation

“…While some of the karyotypic differences arose through the sorting of chromosomal rearrangements that distinguish the parental species, most arose de novo. The natural hybrid species also exhibit increased genome sizes -nearly 50% larger than the parental species [33]. Despite multiple independent origins [34], all species show similar increases in genome size due to the proliferation of retrotransposons [35**].…”

Different scales of Ty1/copia-like retrotransposon proliferation in the genomes of three diploid hybrid sunflower species

“…H. anomalus and H. deserticola inhabit arid desert-like environments, whereas H. paradoxus occurs exclusively in saline environments, such as brackish salt marshes and salt seeps. Estimates of nuclear DNA content using flow cytometry suggest that genome sizes of the hybrid taxa are at least 50% larger than those of parental species (Baack et al, 2005). Variation in genome size among these sunflower species is not attributable to polyploidization because all species are diploid and possess the same number of chromosomes (n ¼ 17).…”

“…PCR efficiency for the standard curve exceeded 96%. Copy number estimates were converted to copy number per genome using mean C values for each species estimated from several populations (Baack et al, 2005). Data from two independent Q-PCR runs were combined to produce Figure 2.…”

“…For the genome equivalent standardization, B1.5 Â 10 5 nucleus equivalents was estimated for each of the five species based on published (Baack et al, 2005) 2C values (H. annuus, 1.10 mg; H. anomalus, 1.76 mg; H. deserticola, 1.63 mg; H. paradoxus, 1.63 mg; H. petiolaris, 1.00 mg). Two PCR probes were used separately in Southern blots: a 230-bp region of the Ty1/copia ribonuclease H (RNase H) domain amplified with non-degenerate primers and a 263-bp region of the Ty1/copia reverse transcriptase (RT) domain amplified with degenerate primers.…”