Akzessorische Chromosomen bei Achillea: Struktur, cytologisches Verhalten, zahlenmässige Instabilität und Entstehung

Summary

• 1. In the polyploid-complex Achillea millefolium L. (sens. lat.) diverse types of accessory chromosomes have been found in populations of the micro-species A. asplenifolia Vent., A. roseo-alba Ehrendf., A. crithmifolia W. K. (all 2x) and A. collina Beck. (4x) (Fig. 1). Cytological analyses of 186 individuals demonstrate that accessories are concentrated on the diploid level (16,3%), rare on the tetraploid (1,2%) and absent on hexa and octoploid levels. Occurrence and frequency of accessories and spontaneous chromosome aberrations are correlated on the different ploidy levels.

• 2. Various steps in the origin of accessory chromosomes have been found in diploid Achillea species: Diverse types of spontaneous chromosome aberrations (including deletions) → heteromorphous bivalents → standard chromosomes and transformed chromosomes as supernumeraries → typical accessory chromosomes, strongly differentiated structurally and without pairing affinity to standard chromosomes (Fig. 1a–c). This problem is further elaborated in the discussion (p. 543).

• 3. Prophase analyses show that accessory chromosomes of A. asplenifolia are about 1/3 the average size of standard chromosomes and have a normal, submedian centromere; they are relatively more heterochromatic than standard chromosomes but have euchromatic segments too (Fig. 2, 3); they differ from standard chromosomes in a number of structural changes. Some derived unstable fragment accessories have been observed (Fig. 4).

• 4. Cytological observations on 325 individuals of A. asplenifolia hybrids and hybrid progenies indicate that the behaviour of accessories during mitosis and meiosis is in some respects different from standard chromosomes.

• 5. In somatic tissues of individuals with increasing primary numbers of accessories (up to 6) there is a growing tendency for intra-individual numerical reduction; only individuals with 2 accessories are relatively stable (Table 1, 2). This reduction is probably due to non-disjunction of accessories and cell competition. The number of standard chromosomes is practically always stable.

• 6. In premeiotic mitoses in the ♂ and especially the ♀ archespor of individuals with the low primary number of 1 accessory chromosome there is a sometimes strong tendency for accumulation of accessories in favour of future PMC's and EMC's (Table 2); this is probably due to non-disjunction and polarized distribution.

• 7. Meiotic pairing and distribution of accessories are quite regular; multivalents are easily formed. Univalents show frequent non-homologous inter-arm-pairing. Accessories never pair with standard chromosomes. Elimination of accessories, even as univalents, is quite low during meiosis (Fig. 5).

• 8. In pollen and ES-mitoses accessories behave normally, non-disjunction is absent (or very rare). There are no indications for special accumulation mechanisms of accessories during fertilization.

• 9. In Achillea the individual dosage of accessories is responsible for their differential cytological behaviour in somatic tissues and the archespor; this in turn is responsible for their direct numerical selfregulation within the population: Increase from low primary numbers, decrease from high primary numbers.

• 10. Normal division versus non-disjunction of accessories may be conditioned by individual euchromatin/heterochromatin balance and its influence on the reduplication of specific heterochromatic segments on the accessories (discussion p. 548–549).