Pairing restriction and non-homologous pairing in autoploids of Adiantum capillus-veneris.

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The high basic chromosome numbers in homosporous ferns are polyploid in origin, and their haploid chromosome complements may contain two or more homoeologous sets of chromosomes (Klekowski 1973). Therefore, chromosome associations in three generations of an autotetraploid of the Venus-hair fern, Adiantum capillus-veneris, have been analysed to test the possibility of a fully penetrant genecontrolled mechanism in the so-called diploid homosporous ferns which limit chromosome associations to only bivalents. Multivalent formation occurs in the autotetraploids, which, though not unexpected, is against a simple gene control to form only bivalents. In the autotetraploid, there are relatively fewer quadrivalents and more bivalents. In the autotriploid, the frequency of trivalents compares closely with those of quadrivalents in the autotetraploids. The occurrence of large number of bivalents in the autotetraploids have been accommodated partly in the model of John and Henderson (1962) which assumes random association of chromosomes within each homologous set. Further accentuation in the number of bivalents occurs partly owing to proximal chiasmata, and partly due to a pairing restriction upon multivalent formation. Following Timmits and Rees (1971), it has been estimated that at least five of the thirty homologous sets in the autotetraploids and autotriploid of A. capillus-veneris associate themselves exclusively in pairs. Chromosome analysis in the autotriploid has also revealed evidence of non-homologous pairing, which seemingly supports the concept of “homoeologous” genomes comprising the haploid complements of extant diploid species in ferns, and also raises the probability of occasional homoeologous pairing and recombination coupled with the pairing restriction.