186 I The Process of Evolution 



Similarly, C. kotschijana (u = 4) was derived from an n = 5 form 

 close to C. joetida. 



There are numerous examples suggestive of progressive increase 

 in chromosome number but unfortunately little experimental evi- 

 dence. The genus Clarkia apparently is one in which chromosomes 

 have been added to the genome. This increase may be associated 

 with the formation of supernumerary chromosomes (see below). 

 Aneuploidy that simulates progressive increase may result when loss 

 or gain of one chromosome is followed by amphidiploidy (doubling 

 of the chromosome number following hybridization of two diploids ) . 

 Part of such a series has been produced in Brassica ( mustard ) where 

 X = 8, 9, and 10 may represent a phylogenetically ascending series. 

 The numbers known in nature or experimentally produced are 

 n = 17, 18, 19, 27, and 29. The genus Carex (sedges) has the most 

 extensive aneuploid series known. Haploid numbers ranging from 

 n = 6 to n = 56 have been reported, and every number from 12 to 

 43 is represented by one or more species. Presumably structural 

 changes and polyploidy have produced some of the numbers in this 

 series. 



Structural rearrangements in which two acrocentric chromosomes 

 give rise to a large metacentric chromosome and a minute fragment, 

 which subsequently disappears, are common in Drosophila, grass- 

 hoppers, and reptiles. The process is known as centric fusion and 

 represents a special sort of reciprocal translocation. In many families 

 or genera, the number of long arms remains constant while the rela- 

 tive number of acrocentrics and metacentrics fluctuates. Many ex- 

 amples could be given. An interesting one is the cricket genus 

 Nemobius. Netnohiiis fasciatus has a metacentric X chromosome and 

 seven acrocentric autosomes. Other species have additional meta- 

 centrics and fewer acrocentrics, presumably the result of structural 

 rearrangements. 



In addition to the basic number of chromosomes in the genome, 

 both plants and animals may have extra chromosomes called super- 

 numeraries. Ordinarily extra chromosomes are not tolerated, for 

 they cause genetic unbalance and upsets in meiosis, as in experi- 

 mentally produced trisomies, tetrasomics, etc. This is why, in gen- 

 eral, only reciprocal translocations can change the basic number. The 

 genetic material remains the same; only its distribution among the 

 centromeres is changed. When supernumerary chromosomes are 

 found, it is evident that they must be neutral in some sense or have 

 a special function. Often they are variable in number from cell to 

 cell, or individual to individual. Nevertheless, it seems unlikely that 

 they are completely inert, since they may in some cases remain in 

 the population. 



