Passage:
Recent investigation into transgenerational epigenetic inheritance in *Arabidopsis thaliana* has refined earlier assumptions regarding how environmental stress induces heritable phenotypic changes without altering DNA sequences. Historically, researchers postulated that stress-induced DNA methylation patterns were uniformly reset during gametogenesis, thereby preventing the transmission of somatic adaptations to subsequent generations. However, contemporary studies demonstrate that specific locus-directed histone modifications and small RNA-mediated chromatin remodeling persistently evade global epigenetic reprogramming during germline development.
Crucially, this persistent epigenetic memory is neither ubiquitous across all stress vectors nor indefinitely self-sustaining. Hyperosmotic stress and nutrient deprivation elicit distinct enzymatic cascades; while osmotic shock frequently recruits CHROMOMETHYLASE 3 (CMT3) to maintain non-CG methylation across vegetative and reproductive lineage boundaries, nitrogen scarcity relies primarily on RNA-directed DNA methylation (RdDM) pathways, which degrade systematically over two clonal generations in the absence of recurring stimuli. Furthermore, the adaptive utility of these inherited marks remains strictly conditional. Under stable environmental regimes, the metabolic expenditure associated with maintaining altered chromatin configurations imposes a measurable fitness cost, manifested as reduced seed viability and delayed flowering. Consequently, natural selection actively suppresses transgenerational inheritance mechanisms except when environmental fluctuations occur within specific temporal thresholds—specifically, cycles shorter than the organismal lifespan but longer than single-generation reproductive windows.
According to the passage, each of the following is true regarding epigenetic modifications and inheritance in *Arabidopsis thaliana* EXCEPT:
- RNA-directed DNA methylation induced by nitrogen scarcity persists indefinitely across generations even in the absence of recurring stimuli.Cevap
- BThe specific enzymatic mechanisms involved in carrying epigenetic memory across reproductive boundaries vary according to the nature of the environmental stressor.
- CRetaining altered chromatin configurations in uniform environments causes a measurable decrease in overall plant fitness.
- DCertain locus-directed histone modifications manage to bypass the comprehensive epigenetic resetting that occurs during germ cell formation.
- ENatural selection favors the transmission of epigenetic marks primarily when environmental changes occur within defined temporal ranges.