BAND AND NUCLEOLAR POLYMORPHISMS IN POLYTENE CHROMOSOMES OF SIMULIUM-ORNATIPES (DIPTERA, SIMULIIDAE)

  • 1 January 1978
    • journal article
    • research article
    • Vol. 21  (82) , 113-133
Abstract
Polymorphisms for the presence or absence of supernumerary bands, for band size, nucleolar expression and the location of secondary nucleolar organizers found in the polytene chromosomes of S. ornatipes are described. In all the 7 band polymorphisms analyzed, 6 of which involve single bands and 1 a multiple band complex (IILH), phylogenetic evidence shows that there has been an addition of material. IILH consists of 7 amplified C[constitutive heterochromatin]-bands, 2 supernumerary C-bands and a supernumerary segment involving 2 C-bands and an interband region. These bands are linked to the paracentric inversion IIL-3, 5 of the bands being located only within inverted segments. Comparison with mitotic chromosomes suggests that IILH heterochromatin is not underreplicated in polytene chromosomes. Recombination between IILH components occurs at a very low level which is insufficient to disrupt the integrity of the polymorphism. The complex apparently evolved in a sequential series, the origin of IIL-3 being the 1st 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 organizers 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.