C-banding pattern and nucleolar organizer regions of amphioxus Branchiostoma belcheri tsingtauense Tchang et Koo, 1936.

Chromosome banding study remains lacking in amphioxus heretofore. This paper reports the C-band pattern of the metaphase chromosomes of amphioxus Branchiostoma belcheri tsingtauense, the first description of banded karyotype in cephalochordate. The C-banding showed that around 50% of the second chromosome pair examined was heteromorphic, while the remaining ones were homomorphic. In contrast, the other 17 pairs were all homomorphic. These broadly support our previous suggestion that the second chromosome pair is a pair of sex-chromosomes. The C-banding stained positively about 54.3% of the chromosome surface. Most of the centromeric and terminal regions in many chromosomes had positive C-bands, and some interstitial C-bands were also observed in some chromosomes. It seems clear that chromosomes of amphioxus are highly heterochromatinized. In addition, silver staining revealed a single pair of nucleolar organizer regions (NORs) located in the telomeric regions of medium-sized chromosomes. It is suggested that the number and location of NORs in amphioxus represent the primitive condition for chordates.