Karyotype Variation and Evolution in Gymnosperms

Abstract

The gymnosperms are the modern representatives of the most ancient group of seed-bearing plants. Their chromosomes have been studied extensively and chromosome numbers are known for representatives of all but three genera. A unique feature of the gymnosperms is the relative uniformity of their chromosome numbers both between species within genera and between genera in families. Polyploidy is very rare. Gymnosperm chromosomes are characteristically large and within most genera karyotypes also show uniformity, the exceptions being amongst some cycads. Despite this overall uniformity of shape and number differential banding patterns with Giemsa and base-specific fluorochromes reveal considerable variation in the number, size, location and base-composition of bands. Similarly, fluorescence in situ hybridization also demonstrates significant variation in number and location of many repetitive DNA elements within and between genera and families. Karyotype evolution does not appear to involve extensive structural rearrangements as meiotic pairing in F1 hybrids is usually regular and where available genetic maps suggest the conservation of large syntenic groups of genes.