Methylated DNA sequence extrusion during plant early meiotic prophase

Reproduction in plants has been thoroughly studied and abundant information is currently available in scientific literature. Here we present new data related to a process of DNA modulation along the meiotic steps, first described 50 years ago, but never pursued further. A time course of DNA content/nucleus by cytophotometric analysis, along the reproductive steps from Pollen Mother Cells (PMCs) to seedlings in barley and oil palm displays a significant modulation of nuclear DNA content that reaches its minimum at the zygote stage. Recovery of the DNA content/nucleus during seed germination stages has been demonstrated. Moreover, cytological and immunocytochemical studies indicate that methylated heterochromatic bodies are extruded from PMC nuclei in the progression of the pre-leptotene stage up to the bouquet. The heterochromatic and methylated status of the extruded bodies gives these data a perspective aligned with exciting current data relating to DNA methylation dynamics and chromatin modifications. Our hypothesis is that somatic cells committed to undergoing meiosis need to release the burden of accumulated somatic multicopy DNA sequences and regain totipotency before entering the process of gametogenesis. The event is interpreted as a mechanism for assuring a single copy DNA pairing and therefore avoiding the risk of unequal crossing-over. Methylation represents the evidence of RNA interference machinery at work, leading to heterochromatinization and extrusion of sequences. The recovery of the extruded DNA sequences along embryo development suggests the occurrence of gene amplification mechanisms.