Unusual germ line limited chromosomes in Acricotopus lucidus (Diptera, Chironomidae)

A small supernumerary polytene chromosome was found during the last 8 years in some rare cases in larval salivary gland cells of Acricotopus lucidus (Diptera, Chironomidae). The chromosome may be derived from the germ line restricted parts of the genome. It consists of a short heterochromatic segment and of euchromatic sections with banding patterns homologous to sections of the short arm of soma chromosome I. When examining male meiosis, an exceptional small germ line limited chromosome was found. It is believed that this chromosome was not always recognized during soma elimination as a germ line limited chromosome, probably because of its partial homology to one of the soma chromosomes, and was then polytenized in salivary gland cells. Another germ line limited chromosome with a characteristic morphology and with a special behavior in differential gonial mitosis was found to have existed for more than 12 years in a laboratory stock. In differential gonial mitosis this special germ line limited chromosome partly pairs with the long arm of soma chromosome I. The present results strongly support the idea that the germ line limited chromosomes of A. lucidus are derived from the soma chromosomes, and show that chromosomes of the germ line restricted part of the genome can persist for many generations in a laboratory stock in spite of complex chromosome elimination mechanisms in the primary germ cells. Key words: germ line limited chromosomes, supernumerary polytene chromosome, salivary gland, Acricotopus lucidus.