THE CHROMOSOMES OF BISEXUAL AND PARTHENOGENETIC SPECIES OFCALLIGRAPHA(COLEOPTERA: CHRYSOMELIDAE) WITH NOTES ON SEX RATIO, ABUNDANCE AND EGG NUMBER

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Abstract
Chromosomes of 31 geographic entities comprising 17 species of Calligrapha were examined. Twelve species are bisexual and have 2n = 22 + X (o) and 22 + XX (o) in both neural and gonial tissue. Spermatocytes contain 11 bivalents and a single X chromosome (MI) and either 11 of 12 daughter chromosomes at ME The chromosome number is modified in some species and some populations by the regular or occasional presence of one or more (up to 12) supernumerary chromosomes. The standard chromosomes are usually metacentric but 2 or 4 may be submetacentric. The auto-somal bivalents are always homomorphic except in C. c. coreopsivora, which has one that is heteromorphic. Aberrant sex ratios in favor of females are found in all but three of the bisexual species. The 1:7 sex ratio of C. scalaris (Ottawa) stems from the occurence of facultative thelytoky. Thelytoky is obligatory in five species. It is accompanied by tetraploidy as 48 chromosomes are present except in the Manhattan form of C. scalaris which has a modified complement of 44. It is considered that parthenogenetic evolution in Calligrapha has been, in order, diploid bisexual reproduction, diploid facultative thelytoky, and finally, tetraploid obligatory parthenogenesis. Bisexual Calligrapha produce about the same number of eggs per season as parthenogenetic females, and thus ''their'' reproductive rate per adult in the population is about 1/2. In nature the bisexual adults are more abundant than the parthenogenetic forms and they are more widely dispersed. Th measure of success is attributed to the advantages of panmixis and chromosomal polymorphism. Evidence is presented for geographic speciation on a single host and on different hosts, and for sympatric speciation in which the essential unit is the colony, or possibly the individual when it becomes parthenogenetic and founds a colony.