- 期刊
玉米B染色體的特性與分子組成
Characteristics and Molecular Organization of the Maize B Chromosome
摘要
B染色體又稱為超數染色體，是一種廣泛存在於許多物種中的非必需染色體。玉米B染色體最早發現於甜玉米中，其存在會影響植株的生長勢。在有絲分裂中期時，玉米B染色體呈現染色較深、短棒狀的末端中節染色體；而在減數分裂粗絲期時，則為近末端中節染色體。玉米B染色體不會和正常A染色體配對，但會影響A染色體的互換頻率。再者，玉米B染色體還具有在花粉第二次有絲分裂時會發生不分離、令帶有B染色體的精核優先與卵受精、及抑制單價B染色體在減數分裂時佚失的特殊遺傳行為；使其能夠在族群中傳遞與累積。這些遺傳行為由B染色體的中節與位於其上的控制因子所控制，同時也受到A染色體的基因影響，但這些控制因子與基因的作用機制仍屬未知。玉米B染色體主要是由與A染色體相似的高度重複性序列所組成，已知的組成序列包含有A染色體的同源基因、反轉位子、及中節之同源序列等；這些序列有些具有B染色體專一或B染色體劑量效應的轉錄能力。另外，玉米B染色體也帶有獨特的專一性序列，利用其中一種位於B染色體三個遠端異染色質區的專一性序列進行系統演化分析，能推測出遠端異染色質區的形成順序；而從玉米B染色體與A染色體共有的中節序列系統演化分析，也能得知玉米B染色體起源於雙中節染色體，二個中節經由不同的演化機制而形成目前的B染色體，然而，玉米B染色體的起源、形成與演化的過程，仍須更多證據才能釐清。
並列摘要
B chromosomes, also called supernumerary chromosomes, are nonessential chromosomes that are widely distributed in many species. In maize, the B chromosome was first found in sweet corn and it affects the vitality of plants. At mitotic metaphase the maize B chromosome is a deeply strained and short-stick-shaped telocentric chromosome. However, at meiotic pachytene the maize B chromosome is an acrocentric chromosome. The B chromosome does not pair with the normal A chromosomes but can affect the crossing-over frequency of A chromosomes. Furthermore, the maize B chromosome displays special behaviors including non-disjunction during the second pollen mitosis, preferentially fertilization of B-chromosome containing sperm nucleus with egg, and suppressing meiotic loss of univalent B chromosome, which allow the maize B chromosome to transmit and accumulate in population. These behaviors are controlled by the centromere and controlling factors on the B chromosome, and simultaneously affected by genes on A chromosomes. However, mechanisms of these controlling factors and genes are still unknown. The maize B chromosome is primarily composed of highly repetitive sequences homologous with A chromosomes, including A chromosome-homologous genes, retrotransposons, centromere-homologous sequences, and so on. Some of these sequences show B-specific or B-dosage transcriptional abilities. Moreover, the maize B chromosome also contains B-specific sequences. By using phylogenetic analysis of B-specific sequences which were located on three distal heterochromatin of the B chromosome, the formation order of the three distal heterochromatin was conjectured. Likewise, phylogenetic analysis of centromeric sequences on both A and B chromosomes indicated that the B chromosome was originated from a dicentric chromosome. The two centromeres went through different evolutionary paths and result in the modern B chromosome. However, more evidence is needed to unveil the origin, formation, and evolution of the maize B chromosome.