Robertsonian translocations and B chromosomes in the Wellington tree weta,Hemideina crassidens (Orthoptera : Anostostomatidae)

Two karyotypes within the species Hemideina crassidens are described, 2n = 15 (XO) and 2n = 19 (XO). These two karyotypes have a NF of 28. The 19-kary otype was found exclusively in the southern part of the species range and t he 15-karyotype was found in the north. The differences between the two kar yotypes are interpreted as arising from two Robertsonian translocations (fi ssion/fusion). Laboratory matings between weta with the two karyotypes prod uced viable offspring. During meiosis in F-1 intraspecific hybrids metacent ric and acrocentric autosomes aligned to form two trivalents, confirming ho mologies predicted by Robertsonian translocations. The subspecies H. c. cra ssiciuris, (confined to Stephens Island) was found to be polymorphic for a metacentric B chromosome. An unusual association of sex and presence of B c hromosome was observed in this island population with Bs found only in male weta.