Bongiorni Silvia, Fiorenzo Paolo, Pippoletti Daniela, Prantera Giorgio
Dipartimento di Agrobiologia e Agrochimica, Università della Tuscia, Viterbo, Italy.
Chromosoma. 2004 May;112(7):331-41. doi: 10.1007/s00412-004-0278-4. Epub 2004 Apr 17.
In the males of lecanoid coccids, or mealybugs, an entire, paternally derived, haploid chromosome set becomes heterochromatic after the seventh embryonic mitotic cycle. In females, both haploid sets are euchromatic throughout the life cycle. In mealybugs, as in all homopteran species, chromosomes are holocentric. Holocentric chromosomes are characterized by the lack of a localized centromere and consequently of a localized kinetic activity. In monocentric species, sister chromatid cohesion and monopolar attachment play a pivotal role in regulating chromosome behavior during the two meiotic divisions. Both these processes rely upon the presence of a single, localized centromere and as such cannot be properly executed by holocentric chromosomes. Here we furnish further evidence that meiosis is inverted in both sexes of mealybugs and we suggest how this might represent an adaptation to chromosome holocentrism. Moreover, we reveal that at the second meiotic division in males a monopolar spindle is formed, to which only euchromatic chromosomes become attached. By this mechanism the paternally derived, heterochromatic, haploid chromosome set strictly segregates from the euchromatic one, and it is then excluded from the genetic continuum as a result of meiotic drive.
在蜡蚧科昆虫（即粉蚧）的雄性个体中，来自父本的一整套单倍体染色体在胚胎期第七次有丝分裂周期后全部变成异染色质。在雌性个体中，两套单倍体染色体在整个生命周期中均为常染色质。与所有同翅目物种一样，粉蚧的染色体是全着丝粒的。全着丝粒染色体的特征是缺乏局部化的着丝粒，因此也缺乏局部化的动粒活性。在单着丝粒物种中，姐妹染色单体黏连和单极附着在减数分裂的两次分裂过程中对调节染色体行为起着关键作用。这两个过程都依赖于单个局部化着丝粒的存在，因此全着丝粒染色体无法正确执行这些过程。在这里，我们提供了进一步的证据，证明粉蚧的雌雄两性减数分裂都是反向的，并且我们提出了这可能是如何代表对染色体全着丝粒状态的一种适应。此外，我们发现雄性个体在减数第二次分裂时会形成一个单极纺锤体，只有常染色质染色体附着在这个纺锤体上。通过这种机制，来自父本的异染色质单倍体染色体组与常染色质染色体组严格分离，然后由于减数分裂驱动而被排除在遗传连续体之外。
