D. D. Shaw

Chromosome differentiation, hybrid breakdown and the maintenance of a narrow hybrid zone in Caledia

A cytogenetic analysis has been performed on hybrids, which include F2 and backcross generations, between two chromosomally differentiated taxa of the grasshopper Caledia captiva. These two taxa differ by a series of pericentric rearrangements which involve seven or eight members of the genome (2n=11II+XO/XX). The chromosomally differentiated taxa form a very narrow hybrid zone in S.E. Queensland in which the average change in chromosome frequency is greater than 50% over a distance of only 200 metres. Hybridisation studies were directed towards explaining both the narrowness of the zone and the role of the chromosomal rearrangements in its maintenance. — The F1 generation does not differ from the parental taxa in its fertility or viability. However, the F2 generation is completely inviable and in the backcross generations viability ranges from 0 to 50%. — The inviability in these hybrids results from embryonic breakdown due to arrested development during embryogenesis. In both the F2 and backcross progeny 10 to 20% of the embryos attain full development prior to hatching but fail to emerge from the egg. The persistence of the thick white cuticle is considered to be the factor responsible for the hatching failure and may be related to a malfunction in chitinase production within the pleuropodia, which normally digests this cuticular layer. — Chromosomal analysis of the segregation patterns of the seven autosomal rearrangements among the viable backcross progeny revealed little evidence of any major differential elimination of the rearrangements to account for the observed levels of embryonic mortality. Rather, it is considered that the mortality is induced by the generation of novel, imbalanced recombinant chromosomes by the F1 parent. Whether or not the unbalanced genotypes arise as a consequence of genic substitution differences accumulated during allopatry or an effect of chromosomal heterozygosity, is difficult to distinguish at this stage. However, an analysis of chiasma distribution in F1 males, which are heterozygous for seven autosomal pericentric rearrangements, has shown that the chiasma pattern in these males is very different to that seen in either parents. It is a possibility that this redistribution of chiasmata is responsible for generating novel genotypes which fail to function during embryogenesis. — In terms of the structure of the hybrid zone, the complete failure of the F2 generation provides an immediate explanation for the observed abrupt change in karyotypic frequency of 50% for each of the diagnostic chromosomes. It is considered that the incidence of embryonic breakdown within the hybrid zone will diminish with time due to the survival of some of the backcross progeny which will gradually reduce the frequency of F1 production. — The observed asymmetry of the hybrid zone cannot be explained from the hybridisation data since there is no evidence of differential survival between backcrosses to the Moreton and Torresian parents.

Population cytogenetics of the genus Caledia (orthoptera: acridinae)

The acridine grasshopper, Caledia captiva exists as two chromosomal races in south-east Queensland. One of these, the “Moreton” race inhabits the coastal region to the east of the Great Dividing Range. All chromosomes of the complement (2n=11II+XO/XX) have been involved in centromeric rearrangement, which transforms the acro- and telocentric chromosomes into submeta- and metacentric elements. The second, or “Torresian” race is widely distributed through southern Papua, Arnhem Land, Cape York Peninsula and down the east coast of Australia as far south as Brisbane. This race, which is characterised by a completely acro- and telocentric chromosome complement, approaches the “Moreton” race in south-east Queensland where the two races are separated by less than 1 km, along a front of at least 150 km. Evidence is presented to show that chromosome introgression is occurring across the contact zone and this takes place in one direction only, namely the “Torresian” chromosomes are infiltrating into the “Moreton” race but not reciprocally. Furthermore, the introgression of chromosomes across the zone is limited to certain members of the Torresian complement and even then these successful chromosomes show highly variable degrees of penetrance into the “Moreton” race. It is proposed that a “tension zone” exists between these two races which is maintained by the interaction of (a) ecological tolerance differences on either side of the zone and (b) by partial competitive exclusion due to the interracial differences in phenology. This case of parapatric association with limited hybridisation is unique in its clarity due to the marked differences in the appearance of the chromosome complements of these races which permits direct assessment of the behaviour of most members of the genome in hybrids and their derivatives.

Allozyme variation across a narrow hybrid zone in the grasshopper, Caledia captiva

SummarySeventeen enzyme systems comprising 23 genetic loci were assayed in the Moreton and Torresian chromosomal taxa Caledia captiva and an assessment was made of the allelic variation within and between these taxa. Heterozygosity per individual in both taxa averaged 10–12 per cent. Neis coefficient of genetic distance between the two taxa was calculated to be 0·158 ± 0·061.Five of the 23 loci showed large differences in frequency between the taxa. ME showed a difference of 50 per cent in allelic frequency, whereas GOT-2, GPT, ICDH-1 and PGI showed differences in frequencies of 90 per cent or greater. Alleles at these loci were characteristic if not absolutely diagnostic of the races.These five enzyme loci provided additional markers for the analysis of samples taken across a transect of the hybrid zone and revealed that the major change in frequency for all loci occurred over a 200 metre interval, which coincided with the location of the major change in chromosomal frequency. However, no gametic disequilibrium between the four characteristic loci could be detected in the central populations of the hybrid zone, implying that these loci, at least, could freely segregate and recombine in hybrids and back-crosses.The highly symmetrical pattern of replacement for these loci contrasts markedly with the very asymmetrical pattern of chromosomal replacement, with high levels of one-way introgression of chromosomes from the Torresian into the Moreton race. Further, the detection of characteristic Moreton alleles, at frequencies of up to 20 per cent, in Torresian populations which contained a very low frequency of introgressed Moreton chromosomes, emphasises that a large proportion of the genome remains undefined and unrecognisable when the hybrid zone is assessed by chromosomal rearrangements alone.

Nuclear DNA variation among acridid grasshoppers

The nuclear DNA amount varies threefold among species of acridid grass hoppers. DNA amount is correlated with the total chromosome volume, as measured at metaphase of mitosis. Despite the large-scale variation in DNA amount and in the total volume of chromosome material there is a striking uniformity in respect of the relative sizes of chromosomes within complements. Males from the northern race of Cryptobothrus chrysophorus contain about 20% more nuclear DNA than males from the southern race. The DNA difference may be explained by supernumerary segments within chromosomes in the northern populations. The magnitude of the DNA variation between these races is indicative of substantial genetic divergence. It may well be that the two races merit separate specific ranking.

REPRODUCTIVE ISOLATION IN RELATION TO ALLOZYMIC AND CHROMOSOMAL DIFFERENTIATION IN THE GRASSHOPPER CALEDIA CAPTIVA

Both chromosomal rearrangements (White, 1969) and the accumulation of small genic differences (Dobzhansky, 1957) have been suggested as possible sources of genetic variation which can initiate reproductive isolation between divergent taxa. However, most investigations of speciation have been limited to correlating allozyme or chromosome differentiation with the level of evolutionary divergence that related taxa have obtained (e.g., Drosophila willistoni group [Ayala et al., 1974], Peromyscus [Zimmerman et al., 1978] and Spalax ehrenbergi [Nevo and Shaw, 1972]). Such an approach reveals little of the underlying mechanisms of reproductive isolation except to illustrate that speciation need not involve allozyme (e.g., Spalax) or chromosomal differentiation (e.g., homosequential species of Drosophila, Craddock and Johnson, 1979). Further, both types of differentiation may be considerable within a taxon and yet reproductive isolation is not evident (e.g., Thomomys bottae, Patton and Yang, 1977). The role of either of these genetic components (chromosomes or genes) has not been adequately examined in any species because of the difficulty of testing hypotheses experimentally. The four chromosomal taxa of the acridine grasshopper, Caledia captiva, however, enable such

Karyotype variation in dermestid beetles

The system of nucleolar controlled sex-chromosome segregation which characterises Xyp species of hide beetles is also present in the one species (haemorrhoidalis) with an XY system. This, coupled with the fact that the karyotype in the XY species is asymmetrical, whereas species with smaller y-chromosomes show greater symmetry, suggests that “erosion” of the y may have involved translocation of the material of the y onto the autosomes rather than simple loss. Finally, supernumerary y chromosomes present in laboratory cultures of two species (maculatus and frischii) demonstrate the efficiency of the sex nucleolus as a mechanism for securing segregation.

Temporal variation in the chromosomal structure of a hybrid zone and its relationship to karyotypic repatterning

A temporal analysis of the chromosomal structure of the hybrid zone in the grasshopper Caledia captiva has revealed that, over a period of six generations, the position of the zone has remained unchanged when assessed in terms of chromosomal frequencies. In complete contrast however, chromosomal genotypic frequencies have changed dramatically and asymmetrically over the same period. The frequencies of chromosomal heterozygotes have been significantly reduced on one side of the zone accompanied by increases in the frequencies of homozygous metacentric chromosomes. These asymmetrical genotypic changes are also reflected in a complete reversal of the patterns of gametic disequilibria (Tr2) across the zone. It is proposed that undirectional selection has favoured a metacentric karyotype on one side of the zone during a major climatic change.The structure of the hybrid zone involves two major and independent features. First, as a secondary consequence of hybridisation, recombinational change in F1 hybrids disrupts the internal organisation within chromosomes. This results in the production of inviable F2 and backcross progeny and hence, explains the structure of the zone in terms of the sharp change in chromosomal frequencies. Secondly, the asymmetrical nature of the gametic disequilibria between chromosomes represents the direction of selection which favours an acrocentric Torresian karyotype in dry years and a metacentric Moreton karyotype during mesic years. Variation in both chromosome structure and embryonic weight is associated with the predictability of the environment. The acrocentric Torresian karyotype and its associated larger embryos are correlated with a univoltine life history in drier, unpredictable habitats. A similar pattern exists within the Moreton subspecies in the form of a chromosomal cline in S.E. Australia. At the southern limit of this cline the karyotype is totally acrocentric, the life history is univoltine and the embryos are the same weight as the Torresian.It is speculated that variation in chromosomal structure, in terms of the relationship between centromeres and telomeres, may provide a mechanism for altering cellular phenotype through changes in such factors as replication patterns or chromatin packaging which may act quite independently of the informational content of the chromosome.

The supernumerary segment system of Stethophyma

Three species of the genus Stethophyma have been cytologically examined and all three show variation both for supernumerary heterochromatic segments and for the distribution of standard heterochromatin among the autosomes. The European species, S. grossum, for example, shows considerable interpopulation variation for standard heterochromatin while two of the populations, from Spain and Austria, show supernumerary segment polymorphism. The segments are located interstitially on the S11 chromosome but occupy different positions in the different populations. — In all species, the presence of the extra heterochromatic segments increases the mean chiasma frequency. Moreover, the influence of the segments upon mean chiasma frequency is different in different populations and in different species. In the Spanish population, the increase is both intra- and interchromosomal whereas in Austria the influence of the segment is completely interchromosomal. — In the American species, S. gracile and S. lineatum, where supernumerary heterochromatic segments are carried on both S10 and S11 chromosomes, the effect on chiasma frequency shows a dosage relationship, an increase in the number of segments per individual being correlated with an increase in mean chiasma frequency. It is suggested that the interstitial segments found in all species have originated by direct duplication of chromosome material. By contrast the terminal segments in S. lineatum and S. gracile may be derived by translocation from a B-chromosome since such a chromosome has been found in one individual of the former species. — The variation in segment structure and the distribution of standard heterochromatin, among the European species of S. grossum suggests that these systems have evolved independently in different populations.

The chromosomal component of reproductive isolation in the grasshopper caledia captiva III. Chiasma distribution patterns in a new chromosomal taxon

SummaryAn analysis of chiasma distribution patterns among two classes of F1 hybrids produced by crossing a new chromosomal taxon, Lakes Entrance (LE), to both the Moreton (MAX) and Torresian (TT) taxa, has demonstrated that, when compared to their parental taxa, the (MAX × LE)F1 hybrids have very different distribution patterns whereas the (LE×TT)F1 hybrids have similar distribution patterns. Chiasmata in the Lakes Entrance and Torresian taxa, and their F1 hybrids generally show proximal-distal patterns of localisation in five of the eight largest autosomes although some subtle statistical differences were detected between the F, hybrids and the parental taxa in those chromosomes. The highly significant differences in chiasma distribution patterns between the (MAX × LE)F1 hybrids and their parental taxa in chromosomes 1, 2, 4, 5, 6 and 8 can be directly attributed to pericentric heterozygosity. In these cases most recombination is localised in the interstitial and distal regions of the chromosomes. Although pericentric heterozygosity would be expected to result in a reduced mean cell chiasma frequency, the (MAX × LE)F1 hybrids have the same mean cell chiasma frequency as both the MAX and TT taxa and the (LE × TT)F1 hybrids. This appears to be due to the presence of exchanges (scored as chiasmata) within the pericentric re-arrangement region. The data strongly suggest that these exchanges are U-type following straight non-homologous pairing at pachytene rather than the result of crossing over following homologous pairing within an inversion loop. In gross stained meiotic material U-type exchanges were in 15·5 per cent of cells scored. The analysis of chiasma distribution in the F, hybrids from crosses between the chromosomally divergent but genically equivalent MAX and LE taxa provides further substantive evidence that the dramatic change in the pattern.of recombination in chromosomally heterozygous F1s disrupts intrachromosomal organisation resulting in the generation of recombinant progeny incapable of completing embryogenesis. In comparison the lack of any noticeable change in the recombination system in the F1 hybrids from crosses between the genically divergent but chromosomally similar taxa, LE and TT, suggests that the F2 inviability in this case is most likely a consequence of recombination between genically divergent genomes involving whole chromosome segregation rather than extensive intrachromosomal recombination.

Comparative chiasma analysis using a computerised optical digitiser.

A new computerised technique has been devised for measuring the distribution of chiasmata along diplotene bivalents. The method involves the introduction into the field of view of the microscope, of a fine light spot which can be accurately manipulated along the chromosomes of each bivalent. The data recorded include (a) the positions of the chiasmata along the bivalent in terms of their relative distances from the centromere and (b) the individual bivalent and cellular chiasma frequencies. — The method has been applied to the analysis of chiasma distribution patterns in the two known species of the genus Caledia, C. species nova 1 and C. captiva and in two chromosomal races of the latter. Statistical tests indicate that within bivalents at least 40% of the comparative distribution patterns of chiasmata between races and species are significantly different. Similar comparisons between populations within races reveal only 18% significant differences. — The observed distribution patterns of chiasmata in this genus suggest that chiasma formation is sequential from centromere to telomere. — The variation in the frequency and distribution of chiasmata between races and species suggests that the interference distances between successive chiasmata are, at least partially, independent of chiasma frequency and position. — The interracial and interspecific differences in chromosome structure are correlated with changes in chiasma pattern.