Meiosis of Allium Porrum, a Tetraploid Species With Chiasma Localisation

W H IL E meiosis of diploid forms with localised chiasmata has been analysed on several occasions within various Liliaceous genera (Fritillaria, Allium, Trillium), this characteristic course of meiosis has not been studied so far in any polyploid form.Such a study affords an approach to problems of very great interest.DARLINGTON (1937) points to one special problem: »In polyploid forms with localised pairing and chiasmata we must expect fewer multivalents than in corresponding forms with complete pairing, since the effective length for pairing is shorter» (1.c. p. 129).This conclusion is drawn from a study of diploid Fritillaria species, where the pachytene pairing in forms with localisation of chiasmata only reaches about 50 % of the total length of the chromosomes.In Allium, on the other hand, the pachytene pairing is morphologically complete in localised species such as Allium fistulosum (LEVAN, 1933), it is complete even in asynaptic species, where no single chiasma is formed.Thus, in an autopolyploid Allium with localised chiasmata a normal frequency of multivalents may be expected at earlier meiotic stages, and this frequency should decrease during the course of the prophase.At metaphase I only such multivalents as are associated by chiasmata should be left, even if the mutual orientation of some bivalents should indicate that earlier they had been joined to quadrivalents.During my work on the cytology of Allium I came across an autotetraploid species, Allium Porrum L., which showed almost complete localisation of chiasmata at meiosis.I studied several forms of the species, both Swedish commercial varieties and material procured from botanical gardens.All these forms were tetraploids and had localised chiasmata.In one form I found a regular occurrence of two small somatic chromosome fragments, which sometimes paired at meiosis, in other respects no cytological differences were found between the different forms.