Repetitive DNA size variation in three sections of the chromosome EF in a population of Chironomus riparius Mg (Diptera, Chironomidae) from Piedmont (Italy)

Abstract

Variation in DNA content was densitometrically analyzed in three sections (E; A4f-g; E; A5d-g; F; B3h) of the chromosome EF of IV instar larvae of Chironomus riparius (syn: C. thummi) Meigen 1804 from a heavy metal polluted Piedmont station (Santena, near Turin). The three sections are known to be composed by AT rich repetitive DNA. Preparations of salivary gland polytenic chromosomes were Feulgen stained and densitometric analysis of digital images of the three sections was performed. DNA density values of each of the three sections fell into several Poisson distributed classes. To explain variation of repetitive DNA content in the three regions, two hypotheses have been considered: during embryonic development, prior to the polytenization process, either somatic mispairing could have occurred with subsequent unequal chromatid exchanges or saltatory replication with subsequent increase of DNA content after breakage of a chromatid. Results support the hypothesis of unequal chromatid exchange as the cause of repetitive DNA size variation.

Key words:

• Chironomus riparius (syn: C. thummi)
• image analysis
• polytene chromosomes
• somatic crossing over
• unequal chromatid exchange
• repetitive DNA