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INTRODUCTION TO CYTOLOGY 



the 30-chromosome hybrid {D. ohovata) showed 10 bivalents and 10 

 univalents in meiosis. Two interpretations of such synaptic behavior 

 were possible: (1) the rotundifolia chromosomes might have paired with 10 

 of those from longifolia (allosynapsis) , leaving the other 10 of longifolia 

 unpaired; (2) the longifolia chromosomes might have paired among 

 themselves (autosi/napsis) because of diploidy in the longifolia gametic 

 complement, leaving the rotundifolia chromosomes unpaired. Subse- 

 quent researches on a number of genera have shown that in some auto- 





B 



Fig. 207. — Chromosome behavior in Rosa hybrids. A, polar view of the chromosomes 

 in the metaphase of the first meiotic mitosis in the mierosporocyte of R. glanca dilatans, 

 showing 7 bivalents surrounded by 21 univalents. B, anaphase of mitosis / in R. glauca 

 plebeia; the bivalents have disjoined and the split univalents lie at the equator. C, late 

 anaphase in R. pomifcra Grenieri. D, division // in R. Junzillii; chromosomes which failed 

 to reach poles in I have formed an extra mitotic figure. E, telophase of // in the same, 

 showing the formation of supernumerary nuclei. F, polyspory (more than 4 spores) 

 resulting from mitotic irregularities in R. pomifera. {After Tackholm, 1922.) 



synapsis, in others allosynapsis, and in still others both types of synapsis 

 may be the rule. Synapsis is generally regarded as an indication of 

 homology in the chromosomes involved, and when used judiciously this 

 criterion is a useful one in the analysis of hybrid chromosome complements. 

 An example of autosynapsis is afforded by 24-chromosome Crepis 

 hybrids formed by crossing C. setosa (4 in gamete) with C. biennis (20 in 

 gamete). In the microsporocytes of this hybrid there are 10 bivalents 

 and four univalents; this indicates autosynapsis among the chromosomes 

 of biennis, a polyploid species. Allosynapsis seems to obtain in the 

 asexually reproducing hybrids in the section Canince of the genus Rosa 

 (Fig. 207). In these hybrids, which have 28 (tetraploid), 35 (pentaploid), 



