Non random distribution patterns of supernumerary segments and B chromosomes in Dichroplus elongatus (Orthoptera)

Dichroplus elongatus is a grasshopper, which exhibits parallel polymorphism s for B chromosomes and supernumerary segments in chromosomes S-10 (SS10), S-9 (SS9) and M-6 (SS6) in natural populations of Argentina. The patterns o f simultaneous variation of different forms of supernumerary heterochromati n from 7 populations of two biogeogaphic provinces located at Northwest and East regions of this country were analysed. Spatial chromosome differentia tion for all heterochromatic variants was observed. Different degree of dif ferentiation among populations for supernumerary segments, evaluated throug h F-ST, was observed. This suggests that genetic drift and migration may no t be the only factors involved in the maintenance of the detected chromosom e patterns. The differentiation for SS9 and SS10 may be explained mainly by heterogeneity within the Northwest Region. On the contrary, the difference s for SS6 and B chromosomes explained through isolation by distance. Howeve r, the absence or the low frequency of both chromosome mutations in some po pulations may not be explained by historical factors. A principal component analysis showed that the patterns of chromosome variation do not agree wit h the geographical distribution of the populations. The relationship betwee n frequencies of each supernumerary segment and B chromosome depends on the segment. An analysis of partial correlation showed that the frequencies of B chromosomes were positively correlated with the frequency of SS6 and neg atively with the frequency of the SS10. In agreement with this, a multiple regression analysis of B chromosome frequency on supernumerary segment freq uencies depends on the incidence of SS6 and SS10. The covariation of the he terochromatic forms may act as another factor that establish limits to stoc hastic factors, may also affect the probable negative selection on B, and c ould be involved in the non random pattern detected in D. elongatus.