Bedo D G
Cytobios. 1978;21(82):113-33.
Polymorphisms for the presence or absence of supernumerary bands, for band size, nucleolar expression and the location of secondary nucleolar organisers found in the polytene chromosomes of Simulium ornatipes are described. In all the seven band polymorphisms analysed, six of which involve single bands and one a multiple band complex (IILH), phylogenetic evidence shows that there has been an addition of material. IILH consists of seven amplified C-bands, two supernumerary C-bands and a supernumerary segment involving two C-bands and an interband region. These bands are linked to the peracentric inversion IIL-3, five of the bands being located only within inverted segments. Comparison with mitotic chromosomes suggests that IILH heterochromatin is not under replicated in polytene chromosomes. Recombination between IILH components occurs at a very low level which is insufficient to disrupt the integrity of the polymorphism. It is concluded therefore that the complex evolved in a sequential series, the origin of IIL-3 being the first step. Single band polymorphisms, some of which are also linked to inversions, show similar heterochromatic properties to the IILH bands. A mechanism of selective DNA sequence amplification is proposed to explain increase in band size and the accompanying heterochromatinization. Most supernumerary bands may be amplifications of submicroscopic bands. Nucleolar organisers show heteromorphism for expression and rare secondary nucleoli are found on all chromosomes. It is argued that a multiplicity of sites for ribosomal genes are distributed in the genome and that selective sequence amplification, similar to that proposed above, can increase these sequences to a functional level at any of the sites. This would explain the lability of nucleolar sites in different blackfly species.
本文描述了在华丽蚋（Simulium ornatipes）的多线染色体中发现的关于额外带的有无、带大小、核仁表达以及次级核仁组织者位置的多态性。在分析的所有七个带多态性中，其中六个涉及单个带，一个涉及多带复合体（IILH），系统发育证据表明存在物质添加。IILH由七个扩增的C带、两个额外的C带以及一个涉及两个C带和一个间带区域的额外片段组成。这些带与近着丝粒倒位IIL - 3相关联，其中五个带仅位于倒位片段内。与有丝分裂染色体的比较表明，IILH异染色质在多线染色体中未发生复制不足的情况。IILH组分之间的重组发生水平非常低，不足以破坏多态性的完整性。因此得出结论，该复合体是按顺序系列进化的，IIL - 3的起源是第一步。单个带多态性，其中一些也与倒位相关联，显示出与IILH带相似的异染色质特性。提出了一种选择性DNA序列扩增机制来解释带大小的增加以及伴随的异染色质化。大多数额外带可能是亚显微带的扩增。核仁组织者在表达上表现出多态性，并且在所有染色体上都发现了罕见的次级核仁。有人认为核糖体基因的多个位点分布在基因组中，并且类似于上述提出的选择性序列扩增可以将这些序列增加到任何一个位点的功能水平。这将解释不同蚋物种中核仁位点的不稳定性。
