Procunier W S, Muro A I
Department of Entomology, British Museum (Natural History), London, England.
Genome. 1994 Dec;37(6):957-69. doi: 10.1139/g94-136.
The Simulium neavei group is medically important as a vector of human onchocerciasis and is unique among blackflies because of the attachment to freshwater crabs during parts of the larval and pupal life cycle stages. Detailed larval salivary gland polytene chromosome maps are presented for two taxa designated S. neavei Amani form A and S. neavei Amani form B, which are probably synonymous with the previously described morphospecies, S. nyasalandicum and S. woodi, respectively. Simulium neavei Amani form B differs from S. neavei Amani form A by three paracentric inversions, a large deletion and a chromocentre, while the latter taxon differs from the former one by a mid-arm interchange. The fixed mid-arm interchange (1-tr) between two related taxa is unique among the Simuliidae. Detailed banding analysis of S. neavei Amani form B shows that, relative to S. neavei Amani form A, sections 16a, 16b, and 17c of the short arm of chromosome one (IS) have been inserted into section 69/70 of the short arm of chromosome three (IIIS). Assignment of directionality for this interchange inferred because of the near universality in the Simuliidae of a chromosome III long arm to short arm ratio of 2:1. We propose that the mid-arm interchange acted as a primary isolating mechanism because of the segregational load on F1 heterozygous interchange progeny. The magnitude of the segregational load would be dependent on the frequency of pairing and crossing-over between the transposed interstitial segment and its original counterpart in the chromosome complement. Nonpairing in the transposed region and normal random segregation would presumably lead to a 50% of gametes carrying the duplicated or deficient transposed segment, resulting in a segregation load of 50%. However, crossing-over in the paired interstitial region may greatly increase segregational load as result of multivalent formation at metaphase one (MI) and the generation of dicentric bridge(s) and acentric fragment(s) upon cell division. Such meiotic disjunction irregularities could lead to meiotic arrest, gametic loss and (or) cell restitution, and the eventual formation of polyploid (lethal) zygotes.
内维蚋类群作为人类盘尾丝虫病的传播媒介具有重要医学意义，并且在蚋类中独一无二，因为其在幼虫和蛹生命周期的部分阶段附着于淡水蟹。本文给出了两个分类单元的详细幼虫唾液腺多线染色体图谱，这两个分类单元分别被命名为内维蚋阿马尼A型和内维蚋阿马尼B型，它们可能分别与先前描述的形态种尼亚萨兰蚋和伍氏蚋同义。内维蚋阿马尼B型与内维蚋阿马尼A型的区别在于三个臂内倒位、一个大的缺失和一个染色中心，而后者与前者的区别在于一个臂间互换。两个相关分类单元之间固定的臂间互换（1-tr）在蚋科中是独一无二的。对内维蚋阿马尼B型的详细带型分析表明，相对于内维蚋阿马尼A型，一号染色体（IS）短臂的16a、16b和17c节段已插入三号染色体（IIIS）短臂的69/70节段。由于蚋科中三号染色体长臂与短臂的比例几乎普遍为2:1，因此推断出这种互换的方向性。我们认为臂间互换作为一种主要的隔离机制，是因为F1杂合互换后代存在分离负荷。分离负荷的大小将取决于转座的间质片段与其在染色体组中原始对应片段之间配对和交换的频率。转座区域的不配对和正常的随机分离可能会导致50%的配子携带重复或缺失的转座片段，从而产生50%的分离负荷。然而，配对的间质区域中的交换可能会由于中期一（MI）的多价体形成以及细胞分裂时双着丝粒桥和无着丝粒片段的产生而大大增加分离负荷。这种减数分裂分离异常可能导致减数分裂停滞、配子丢失和（或）细胞恢复，并最终形成多倍体（致死）合子。
