Supernumerary Chromosomes of some Species ofSolidagoand a Related Taxon

Abstract

SUMMARYThe author present a study of the distribution, frequency, and behaviour of the supernumerary chromosomes found in populations of 12 species of the genus Solidago and one closely related taxon, Euthamia graminifolia, all of the family Compositae, tribe Asterieae.The supernumerary chromosomes are numerically variable. Their number ranges between 0 to 4. The number is not constant for a given plant or a species. Strikingly, these chromosomes are not found in dividing pollen mother cells in the anthers.Supernumeraries are euchromatic in 10 of the taxa namely: Euthamia graminifolia var. Nuttallii, Solidago aestivalis, S. bicolor, S. hispida, S. macrophylla, S. nemoralis, S. puberula, S. rugosa forma villosa, S. squarrosa and S. uliginosa but tend to be heterochromatic, particularly at prometaphase in the 3 species: S. canadensis, S. juncea and S. sempervirens.In their morphology, supernumeraries are mostly shorter than or equal to the short arms of some of the normal chromosomes, or resemble the satellite chromatin knobs. They are globular, rectangular or irregular and appear to be acentric or, probably, to bear terminal centromeres.Meiotic chromosomes generally show regular bivalent formation and a normal disjunction at anaphase. The range in pollen fertility (= viability) is very high in all investigated taxa except in a population of S. juncea which may contain hybrid plants. This high degree of pollen fertility in the majority of taxa and generally regular meiosis indicates that supernumeraries have no apparent adverse effect on sexual reproduction in these plants.On the basis of their morphology it is suggested that supernumerary chromosomes probably arise here by disarticulation at the weak secondary constrictions in the satellite chromosomes, by misdivision of a primary constriction in non-satellited chromosomes or by reciprocal translocations. Alpha-bromonapthalene used in pretreatment should be discounted as an agent as these chromosomes are present in both treated and untreated material.