Chromosomal variation in Agromyzidae (Diptera)

The polytene chromosomes of nine species were investigated and chromosome maps are presented. Together with four earlier investigated species, two of which consist of two semispecies each, they are compared as to chromosomal relationships. The basic chromosome number 2n = 12 has been reduced by one centric fusion in one semispecies and by three centric fusions in three species each. The chromosome number has increased through polyploidy in the parthenogenetic species Phytomyza crassiseta to 2n = 18. In species with 2n = 12, five rods and one dot occur in five species, six rods occur in four species (or semispecies) and one V and five rods occur in one species. In polytene chromosome cells there are always five euchromatic major arms and (as a rule) one very small one. The relative lengths of chromosome arms are similar within genera. Intra-arm-rearrangements have resulted in a great dissimilarity among species as to band sequences. In two groups of species it has been possible to homologize the chromosomes and to estimate the number of breaks lying behind species differentiation. One of these groups included three and the other two species. Chromosomal polymorphism occurred in five species. Structural rearrangements, fixed in heterozygous condition, occurred in the parthenogenetic species. Inversions, fixed in homozygous condition, (incipient species differentiation) occurred in two species. Chromosomal evolution seems to have been due to paracentric inversions, changes in single bands and increase in the amount of constitutive heterochromatin.