Cytogenetic studies on two rubus arcticus-hybrids

Authors

• Antero Vaarama State Horticultural Institute, Piikkiö, Finland

DOI: https://doi.org/10.23986/afsci.71255

Abstract

The subject of the study has been the F1- and F2-generations of an artificial diploid Ruhus idaeus x arcticus hybrid and the regeneration of a spontaneous triploid R. saxatilis x arcticus hybrid. In these the meiosis in the PMC has been studied. In the F2-generation of the former hybrid observations on the inheritance of a number of species characters have been made. The meiosis of the F1-generation in R. idaeus x arcticus is regular. In 83.4 % of the divisions chromosome pairing is complete and 7 bivalents are formed. In 10.6 % of the cells 6 bivalents and 2 univalents are observed. In 6.0 % of the cells the division of one bivalent was irregular. In the meiotic divisions of the F2-generation no disturbances were observed. In the meiosis of R. saxatilis x arcticus varying numbers of univalents, bivalents and trivalents are seen. The trivalents sometimes form inversion bridges and are delayed in their division. Some chromosomes are consequently eliminated and the chromosomes are divided unequally between the poles. The second division is more regular; sometimes, however, a few chromosomes are eliminated. The F1-generation of R. idaeus x arcticus now studied is very sterile. In part this sterility depends on chromosomal irregularities. Mainly, however, it is caused by the fact that the anthers and the pollen suffer from drought during the summer and consequently no fertilization can take place. In autumn when the humidity of the air becomes higher, some fruit formation takes place. This sterility is here called seasonal sterility and it is thought to depend on an unfavourable gene combination formed by the idaeus and arcticus chromosome complements. In the F2- generation fertility is restored owing to the breakdown of this combination. R. saxatilis X arcticus is completely sterile owing to irregular meiosis. In the F2generation of R. idaeus x arcticus a strong segregation of the species characters takes place. The observations indicate that the two species have a number of allelomorphs in common. It is assumed that the inheritance of some characters is connected with the action of several modifier genes. It has been established that R. arcticus belonging to the Cylactis section of the genus Rubus and R. idaeus belonging to the Idaeobatus section have been derived from common ancestral forms at some time before the pleistocene period. The differentiation has probably been caused by a gene or chromosome mutation leading to seasonal sterility. Speciation has then continued mainly through gene mutations. Rearrangements in chromosome structure have been so small that the ability of the chromosomes to conjugate has not been changed.