BIOSYSTEMATIG STUDY OF SOME GLYCERIA SPECIES -II 305 



Discussion 



The origin of G. X pedicellata 



The high univalent frequency at meiosis in this species fully accounts for the complete 

 sterility observed, and confirms the view, generally accepted on grounds of morphological 

 intermediacy, that G. X pedicellata is a hybrid between G. fluitans and G. plicata. 



G. X pedicellata is a perfect example of a successful vegetatively propagated hybrid 

 which spreads extensively by means of stolons, forming large stands which often occur in 

 swift flowing streams. Under these circumstances, dispersal can take place by detached 

 ramets carried by the moving water. The vegetative efficiency of this plant is such that 

 it is often found in places from which one or both parental species are absent. 



The status of sterile G. plicata. HA26 



Meiotic pairing in panicles from various parts of this sterile clone was entirely different 

 from that observed in G. X pedicellata. The majority of the chromosomes paired as 

 bivalents but in addition 6-valents occurred involving associations of unlike-sized and 

 therefore presumably non-homologous chromosomes (Fig. 9). This is consistent with 

 the view that a segmental interchange has taken place between non-homologous chromo- 

 somes (Darlington 1937). This meiotic irregularity does not seem sufficient in itself to 

 account for the complete sterility observed, which suggests that some further genetic 

 and physiological disturbances are present. This plant should be regarded as a sterile 

 form of G. plicata. 



Interspecific Relationships 



In section Glyceria there are two types of plant in which a study of metaphase pairing 

 should provide evidence of the relationships between the genomes involved. These are 

 the hybrids between G. declinata and the two tetraploids, and the hybrid between the 

 tetraploids. Attempts to synthesize these hybrids have not proved successful and no 

 spontaneous triploids have been found. This will be described in a subsequent paper. 

 Discussion is therefore confined to the naturally occurring hybrid G. X pedicellata and 

 its parents. 



A detailed analysis of pairing in G. X pedicellata reveals some affinity between the 

 parents, since on average 10-7 bivalents per cell occurred, and hence there appears to be 

 some homology between one genome from each parent. The maximum number of 

 bivalents per cell observed was fifteen, however, and this implies either that intragenomal 

 pairing occurs, or, more probably, that there is some homology between chromosomes 

 from all four genomes. The occurrence of about one trivalent per cell and the rare 

 occurrence of a quadrivalent support the latter view. 



A study of pairing in the parents may assist in understanding the nature of pairing 

 in the hybrid. The low chiasma frequency per bivalent, 1.13 in G. plicata and 1-34 in 

 G. fluitans, shows that in many chromosomes a chiasma is formed in one arm only, this 

 may be associated with the small size of the chromosomes, and would permit little multiva- 

 lent formation to occur; nevertheless, quadrivalent formation occurs sporadically in some 

 populations of G. plicata although this species has the lower chiasma frequency, and it 

 is therefore possible that G. plicata is an autopolyploid or segmental allopolyploid (Stebbins, 

 1947). In G. fluitans, with a higher chiasma frequency, bivalent pairing is the rule, 

 suggesting that this species may be allopolyploid. 



The evidence suggests that in G. X pedicellata there is homology between one genome 

 from each parent, some homology between these genomes and the second genome of 

 G. plicata, and slight homology between some chromosomes from one or more of these 



