S. WALKER 



205 



D.CWSTATA. 



. D I L ATATA . i^Sl t £2^ 



• D.SPINULOSA. 



D. DIL ATATA. 



Fig. 11. Hybridisation within the complex. Continuous lines show wild hybrids and broken lines those 

 synthesised. Numbers of bivalents formed during meiosis are shown in brackets; other numbers represent 



the univalents formed. 



gametic chromosome set present in a species-hybrid still remain. The available evidence, 

 however, suggests that D. cristata, D. spinulosa and D. dilatata sue in fact allotetraploids. 



In Eastern North America and Canada D. cristata is involved in a number of hybrids 

 which have been cytologically examined (Manton & Walker, 1953; Walker, unpublished). 

 Two of these are triploid and show complete failure of pairing at meiosis but another 

 is tetraploid with pairs and univalents; bivalents are not formed therefore where there 

 is a lack of chromosome homology between the parent species and this excludes auto- 

 ploidy in D. cristata. 



Evidence against autoploidy in D. dilatata has been provided by an investigation 

 of induced apogam.y in this species (Manton & Walker, 1954). An apogamous sporophyte 

 with a somatic complement of 82 (normal sexually reproduced sporophytes have 2n=^164) 

 shows complete failure of pairing between these chromosomes during meiotic division. 



For D. spinulosa the evidence is less complete. This species does not, so far as is 

 known, form hybrids, which show complete failure of meiotic chromosome pairing, nor 

 has the formation of an apogamous plant yet been induced. Reference to Fig. 11 however, 

 will indicate the variation in the number of bivalents (33 to 39) formed in hybrids involving 

 D. spinulosa. If this species were an autotetraploid, the number of bivalents, arising 

 from homologous pairs within the gametic chromosome set, should remain more constant 

 in the different hybrids. 



The D. spinulosa complex includes, on available evidence, three allotetraploid species 

 inter-related by common diploid ancestors. If the genomes of the ancestral diploids are 

 represented by letters, the genetic constitution of the three allotetraploids can be illustrated 

 as below : 



D. cristata 

 D. spinulosa 

 D. dilatata 



A + B 



B + C 



C + D (or B + D, or C + A) 



