The number of chiasmata in bivalents and the behaviour of chiasmata during the meiotic divisions were studied in Psylla foersteri (Psylloidea, Homoptera). Two chiasmata with a frequency of 97% and one or three chiasmata with frequencies of 2% and 0.9%, respectively, were observed in the largest bivalent in male meiosis. Meiosis was normal for the largest bivalents with one or two chiasmata, whereas bivalents with three chiasmata were not capable of completing anaphase I because of their inability to resolve the chiasma located in the middle. Consequently, the bivalent was seen as a laggard joining together two metaphase II daughter plates. Apparently, cells of this kind are eliminated. Inability to resolve the chiasma situated in the middle is attributed to the condensation process, which is unable to change the spatial orientation of successive chiasma loops in holocentric bivalents so that chiasma loops would be arranged perpendicular to each other at metaphase I. Thus they retain their parallel orientation from diplotene to metaphase I. Consequently, sister chromatid cohesion is exposed for release only in the outermost chiasmata but the chiasma in the middle continues to interlock the chromosomes in the bivalent. The elimination of the cells carrying bivalents with more than two chiasmata creates a strong selection against the formation of more than two chiasmata in holocentric bivalents.

The Cimicomorpha is one of the largest and highly diversified infraorders of the Heteroptera. This group is also highly diversified cytogenetically and demonstrates a number of unusual cytogenetic characters such as holokinetic chromosomes; m-chromosomes; multiple sex chromosome systems; post-reduction of sex chromosomes in meiosis; variation in the presence/absence of chiasmata in spermatogenesis; different types of achiasmate meiosis. We present here a review of essential cytogenetic characters of the Cimicomorpha and outline the chief objectives and goals of future investigations in the field.

NOKKALA, S . and NOKKALA. C . 1983. Achiasmatic male meiosis in two species ofSaldula (Saldidae, Hemiptera). -Hereditas 99: 131-134. Lund, Sweden. ISSN 0018-0661. Received November 29, 1982 Male meiosis in two species o f Saldula, S. orthochila and S. salfatc~ria was studied. The diploid chromosome numbers were found to be 2n=32+m+XY and 2n=32+2m+XO respectively. The existence of a Y-chromosome was recorded for the first time in this primitive heteropteran genus. The meiotic behaviour o f the sex chromosomes was found to be similar to that o f sex chromosomes in more advanced Heteroptera. Autosomal bivalents during late meiotic prophase and metaphase I consisted o f two parallelly aligned chromosomes. No traces o f chiasmata could be found. The role o f Y-chromosome in the evolution o f sex chromosome systems in Heteroptera, the mechanisms behind the regular segregation o f achiasmatic chromosomes, the adaptive significance of achiasmatic meiosis and the restriction of achiasmatic meiosis to the heterogametic sex are discussed. Skorpionsfliege Panorpa (Mecoptera). -Cliromosomn 12: 2 15-232 WELSCH, B. 1973. Synaptonemal Complex und Chromosomenstruktur in der achiasmatischen Spermatogenese von Panorpa communis (Mecoptera). -Chromosoma 43: 1%74 WHITE, M. J. D. 1%5a. Chiasmatic and achiasmatic meiosis in African Eumastacid grasshoppers. -Chromowrna 16: 271-307 WHITE, M. J. D. l%Sb. Sex chromosomes and meiotic mechanism in some African and Australian mantids. -Chromosoma 16: 521-547 WHITE, M. J. D. 1973. Animal cytology and evolution (3 ed.). -Cambridge (Jniversity Pres.5 , Cambridge

Using a cytological approach, diploid females were found coexisting with rare males in triploid apomictic parthenogenetic populations of the psyllid Cacopsylla myrtilli (W. Wagner, 1947) in Norway, Sweden, Finland and northwest Russia. Diploid females were easily distinguished from triploid apomictic females by the presence of 13 chiasmate bivalents instead of 39 univalent chromosomes at metaphase I. Abundance equaled that of males, but the proportion of males and diploid females was significantly greater in high altitude compared with low altitude populations. Males mated with females but showed no mating preference for diploid females. Lack of genuine bisexual reproduction owing to either asynaptic meiosis in males, or rarity of males with normal meiosis, suggests that diploids are produced in every generation by parthenogenetic females as reversals from triploidy, with their production being enhanced by environmental factor(s) associated with high altitude. This is further supported by the observation that within a population the COI haplotype found in rare males was the same as that in parthenogenetic triploid females. Thus, in northern Europe parthenogenesis in C. myrtilli is obligate, geographic parthenogenesis. Bisexual populations of C. myrtilli should be looked for in Central and Southern Europe. From the evolutionary point of view, the presence of males and diploid females with normal meiosis in parthenogenetic populations could be significant as they exhibit the potential to re-evolve either a new sexual species of parthenogenetic ancestry or a new parthenogenetic species by contagious parthenogenesis.

For studying meiosis in males, large samples of Cacopsylla myrtilli (Wagner, 1947) (Hemiptera, Psyllidae) were collected in Norway, Sweden, Finland and northwest Russia. In addition to all-female populations, males were present in 10 out of 47 populations; still, all populations were highly female-biased, the proportion of males varying from 0.1% to 9.1%. These males are thus rare or so-called spanandric males. Males in northern Norway, Finland and northwest Russia showed normal chiasmate meiosis, while complete absence of chiasmata due to asynapsis was found in males collected in Norway and northern Sweden. In asynaptic meiosis, all univalent chromosomes divided during the first meiotic division resulting in incomplete second meiotic division and formation of diploid sperms. Hence, males in these populations are nonfunctional and do not contribute to the genetic constitution of the population, but appear in every generation as reversals from apomictic parthenogenesis and the mode of parthenogenesis is of obligatory type.