Since the discovery of B chromosomes, multiple different definitions of these selfish genetic elements have been put forth. We reconsidered early definitions in light of recently published studies. While there are many characteristics that vary among different B chromosomes, such as their evolutionary origins, size, segregation behaviors, gene content, and function, there is one defining trait of all B chromosomes: they are nonessential for the organism. The points raised here may be useful for framing future B chromosome studies and help guide the categorization of new chromosomal elements that are uncovered in genomic studies.

中文翻译：

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什么是B染色体？重新审视早期的定义

自从发现B染色体以来，人们对这些自私遗传元件提出了多种不同的定义。我们根据最近发表的研究重新考虑了早期的定义。虽然不同 B 染色体之间有许多特征不同，例如它们的进化起源、大小、分离行为、基因内容和功能，但所有 B 染色体都有一个定义性特征：它们对于生物体来说不是必需的。这里提出的观点可能有助于构建未来的 B 染色体研究，并有助于指导基因组研究中发现的新染色体元件的分类。