C banding in polytene chromosomes of Simulium ornatipes and S. melatum (Diptera: Simuliidae)

Abstract

Polytene and mitotic chromosomes of Simulium ornatipes and S. melatum were subjected to C banding procedures. In both species polytene chromosomes consistently show C banding of centromere regions, telomeres, nucleolar organiser and, unexpectedly, numerous interstitial sites. The interstitial C banding sites correspond to morphologically single polytene bands. Their response is graded and independent of band size. Interstitial C bands in S. ornatipes are scattered throughout the complement, whereas in S. melatum they are clustered. Supernumerary heterochromatic segments in S. ornatipes also exhibit strong C banding and inverted segments can differ from standard in C banding pattern. -- Mitotic chromosomes of both species show a single centric C band with indications of two weak interstitial bands in S. ornatipes, suggesting that many C band regions, detectable in polytene chromosomes, are not resolved by present techniques in mitotic chromosomes. -- Contrary to current opinion that C banding is diagnostic for constitutive heterochromatin, the interstitial C band sites of polytene chromosomes are regarded as euchromatic. Conversely, the heterochromatic pericentric regions of S. ornatipes are not C banded. -- It appears that polytene chromosomes offer a promising system for the elucidation of C banding mechanisms.

CYTOGENETICS AND EVOLUTION OFSIMULIUM ORNATIPESSKUSE (DIPTERA: SIMULIIDAE). II. TEMPORAL VARIATION IN CHROMOSOMAL POLYMORPHISMS AND HOMOSEQUENTIAL SIBLING SPECIES

“…At each locality samples were taken from the same 10 to 30 m length of the stream. Methods for the collection, transportation and fixation of larvae; and polytene chromosome slide preparation have been described previously (Bedo, 1975(Bedo, , 1977 Larvae were scored for inversion and band polymorphisms and the chromosomal constitution of each individual was coded on computer punch cards for use as input data with the library program "Statistical Package for the Social Sciences" (SPSS). Only common polymorphisms, those reaching frequencies greater than 5%, were coded.…”

“…The morphospecies S. ornatipes has two sibling species, S. ornatipes A and B, distinguishable by a fixed inversion difference, differentiated sex chromosomes, and unique floating inversions (Bedo, 1977). S. ornatipes A is very widespread, occurring in all Australian states and on Norfolk Island.…”

Chromosomal characteristics and evolutionary relationships of the Palearctic black fly Simulium carthusiense (Diptera: Simuliidae)

“…Secondary nucleolar organizers in the Simuliidae are frequent across species, but infrequent within species. Multiple nucleolar sites are postulated to occur in the genome, with one dominant to the exclusion of others in a species; incomplete suppression, however, might result in nucleolar expression at another site (Bedo, 1978).…”

“…Of the two heterobands, only IIIS Hb-74C2 occurred in more than one larva. Heterobands, reflecting enhanced DNA content, are found in many species of the Simuliidae (Bedo, 1978), including other members of the S. vernum group (Hunter, 1987). None of the rearrangements in S. carthusiense were linked to the X or Y chromosomes; the sex chromosomes, therefore, were microscopically undifferentiated (X 0 Y 0 ).…”

Multiple sex determining regions in a group of related Chironomus species (Diptera:Chironomidae)

“…Beermann (1955) noted that in the presence of polymorphic Y determiners the male population was split up in terms of sex determination since there was no way in which a Y determiner could transfer to a male carrying the alternative Y. Several authors have noted the tendency in both chironomids and simuliids, for the accumulation of inversions, or other mechanisms to reduce crossing over, in the vicinity of the sex determining regions (e.g., Beermann, 1955;Rothfels, 1956;Bedo, 1975Bedo, , 1979. This could mean that a fairly large segment of the chromosome is tied to the sex determiner and hence incapable of transmission to males carrying an alternative Y chromosome.…”