Date Published: February 3, 2017
Publisher: Public Library of Science
Author(s): Semen M. Bondarenko, Gleb N. Artemov, Igor V. Sharakhov, Vladimir N. Stegniy, Immo A. Hansen.
Spatial organization of chromosome territories is important for maintenance of genomic stability and regulation of gene expression. Recent studies have shown tissue-specific features of chromosome attachments to the nuclear envelope in various organisms including malaria mosquitoes. However, other spatial characteristics of nucleus organization, like volume and shape of chromosome territories, have not been studied in Anopheles. We conducted a thorough analysis of tissue-specific features of the X chromosome and nucleolus volume and shape in follicular epithelium and nurse cells of the Anopheles atroparvus ovaries using a modern open-source software. DNA of the polytene X chromosome from ovarian nurse cells was obtained by microdissection and was used as a template for amplification with degenerate oligo primers. A fluorescently labeled X chromosome painting probe was hybridized with formaldehyde-fixed ovaries of mosquitoes using a 3D-FISH method. The nucleolus was stained by immunostaining with an anti-fibrillarin antibody. The analysis was conducted with TANGO—a software for a chromosome spatial organization analysis. We show that the volume and position of the X chromosome have tissue-specific characteristics. Unlike nurse cell nuclei, the growth of follicular epithelium nuclei is not accompanied with the proportional growth of the X chromosome. However, the shape of the X chromosome does not differ between the tissues. The dynamics of the X chromosome attachment regions location is tissue-specific and it is correlated with the process of nucleus growth in follicular epithelium and nurse cells.
Interphase chromosomes maintain integrity and occupy specific volume known as chromosome territories (CTs) inside the nucleus [1–2]. Non-random organization of CTs is important for the functioning of the genetic apparatus of the cell . A significant aspect of the nuclear architecture is interaction between chromatin and other nuclear compartments. For example, lamina plays a fundamental role in the process of CT formation at the nuclear periphery . The nucleolus is a ribosomal RNA synthesis center, which is formed by nucleolus organizer regions (NORs) localized on acrocentric chromosomes of humans  or on the X chromosome of fruit flies  and mosquitoes . CTs that contain NORs usually localize near the nucleolus or associate with it . Other chromosomal regions besides NORs known as nucleolus associated domains (NADs) may contact with the nucleolus as well .
Principles of the 3D genome organization must be thoroughly studied in vector species because of possible dynamic changes in the nuclear architecture upon infection with a pathogen . Previously we demostrated tissue-specific features of the spatial chromosome organization in An. messeae based on data obtained by manual geometrical measurements of only two points for every nucleus . In this work we have shown that the methods of the chromosome spatial organization analysis using TANGO is applicable to studying the shape and size of polytene chromosomes and chromosome dynamics during nucleus growth. The movement of the longitudinal axis of the X chromosome with the change of the nucleus size is likely associated with the change in the number of NE-attachment regions. This idea agrees well with the data obtained by modeling of Drosophila salivary gland nuclei [23–24]. The tissue-specific differences of the dynamics of the X chromosome and NE-attachment regions could result in gene expression differences [1, 10, 13]. Spatial characteristics of the X chromosome and nucleolus in An. atroparvus will serve as a baseline for similar studies in other species from the Maculipennis complex to which An. atroparvus belongs. Future studies will also address species-specific aspects of the nuclear architecture. A study of the evolution of the nuclear architecture will assess the possibility of using some features of 3D genome organization as markers for understanding phylogenetic relationships within the species complex.