P. Wilkinson

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.

Gene flow across a chromosomal tension zone. I: Relicts of ancient hybridization

Two parapatric subspecies of grasshopper with extensive karyotypic differences form a hybrid zone in which the change-over of chromosomal characters occurs over a distance of 800 m. Asymmetrical introgression of restriction-fragment markers of the nuclear ribosomal RNA genes, and mitochondrial DNA, and also of four enzyme electromorphs is reported. These markers were found to have introgressed for varying distances (100–300 km) to the north of the present-day hybrid zone. It is proposed that these markers are relicts of ancient hybridization between the Moreton and Torresian subspecies in an area where only the Torresian form (as defined by karyotype) is now found, and that the two taxa have maintained their chromosomal distinction despite prolonged hybridization and the geographic displacement of the Moreton subspecies by the Torresian form.

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.

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

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.

“Homologies” between non-homologous chromosomes in the grasshopper Caledia captiva

Cytological studies of hybrids between three chromosomal forms of the grasshopper, Caledia captiva, have revealed a clear case of pairing and exchange between non-homologous chromosomes. The genomes of each of the three chromosomal forms are readily identifiable by their marked differences in morphology and in the pattern of C-heterochromatin distribution. The testes of inter-racial F1 hybrid males contain both diploid and tetraploid meiocytes within the same individual. Multiple chromosome associations are a regular feature of all diploid cells. In many cases, these multiples involve two or more non-homologous chromosomes from within the same haploid genome. Such associations reveal unambiguous evidence of meiotic exchange and “chiasmata”. The X chromosome is frequently observed to associate with an autosome, and anaphase I cells provide evidence of X/autosome exchanges. A correlation exists between the position of the exchange event in non-homologous pairs and the location of heterochromatin. In tetraploid meiocytes, pairing is by strict homology only, giving rise to cells with 22 bivalents plus an XX bivalent or two univalent X chromosomes. Segregation patterns in tetraploid cells are entirely normal and result in the production of diploid gametes. In the male, the increased ploidy level was observed to arise following an endoreduplication process which takes place pre-meiotically in the spermatogonial cells. The finding that non-homologous chromosomes from within the same haploid genome can pair and cross over during meiosis clearly shows that some caution must be taken when interpreting multiple associations as evidence of interchange heterozygosity in hybrids.

A comparison of chromosomal and allozymal variation across a narrow hybrid zone in the grasshopper Caledia captiva.

A hybrid zone between the Moreton and Torresian taxa of the grasshopper Caledia captiva in S.E. Queensland has been characterised in terms of allozyme and chromosome variation within the same individuals. — On chromosomal criteria (pericentric rearrangements), the zone is asymmetrical with evidence of high levels of introgression of Torresian chromosomes into the Moreton taxon. This is apparent from the analysis of two independent transects across the hybrid zone. Major changes in chromosomal frequency occur over distances of less than 0.5 km. and the level of introgression differs between the two transects, with much higher levels in the northern Moreton populations, characterised by an acrocentric X-chromosome, when compared with the southern metacentric-X Moreton populations. Chromosome analysis of samples taken from the same transect over two years has revealed no major changes in the structure of the zone. Moreover, a Moreton population located only 0.5 km. from the null point was found to be stable over 6 generations with evidence for a new balanced genome having originated following the differential incorportation of Torresian chromosomes. — Contrary to the chromosomal situation, the same hybrid zone was found to be symmetrical with respect to allozyme variation with evidence of movement of diagnostic alleles in both directions across the zone. The alleles are independent and not tightly linked to any of the pericentric rearrangements. Thus these 5 alleles are acting as markers of the background genome and reveal the relatively free movement of genes which are located outside the pericentric rearrangements. — It is proposed that the hybrid zone in Caledia captiva is unstable and is moving slowly in a westerly direction into the Torresian territory. This is due to the ability of the Moreton taxon to incorporate more readily into its genome those Torresian chromosomes or chromosome segments which increase the fitness of the Moreton taxon. On chromosomal criteria, the Torresian taxon does not share the same capacity. — It is suggested that, so long as the two taxa retain their ability to hybridise with subsequent asymmetrical introgression, the zone will continue to move westwards and eventually lead to the selective incorporation of the Torresian genome into the Moreton taxon. This will result in a polymorphic situation with clinal variation in chromosomal frequencies. The structure of the zone is dependent upon a fine balance between genomic reorganisation in recombinant genotypes and the relative dispersal capacities of the two hybridising taxa.

The chromosomal component of reproductive isolation in the grasshopper Caledia captiva

A comparison of chiasma distribution patterns between two chromosomal taxa, Moreton and Torresian, and their F1 hybrids demonstrates highly significant differences between all chromosomes analysed. In chromosomes 1, 2, 4, 5, 6 and 8 these differences can be directly attributed to pericentric heterozygosity in the F1 hybrid. In chromosomes 7 and 8 where there is no pericentric heterozygosity these differences may be due to heterozygosity for interstitial and terminal bands of hetero-chromatin or possibly undetected paracentric rearrangements. The F1 hybrids also have a significantly lower mean cell chiasma frequency. The Moreton and Torresian taxa differ significantly in chiasma distribution pattern in chromosomes 1, 2, 4, 5, 6 and 8 and both Moreton populations analysed have a significantly lower mean cell chiasma frequency than the Torresian population. In addition the two Moreton populations, (MMX) and (MAX), differ significantly in the chiasma distribution pattern in chromosomes 1 and 2 and the chromosomally more polymorphic population (MMX) has a significantly lower mean cell chiasma frequency. There is some evidence that the differences in both chiasma distribution and frequency between these two populations may be due to genetic differences in addition to the effects caused by chromosomal polymorphism. It has been shown that in general there is a substantial reduction in recombination in the intersitial regions of most chromosomes in the Moreton and particularly the Torresian taxon because of a proximal-distal localisation of chiasmata. In the F1 hybrid, however, nearly all recombination events are located within these interstitial regions. This provides support for the hypothesis that the frequent placement of chiasmata in regions of normally low recombination may disrupt the internal coadapted genetic environment of both chromosomal forms resulting in non-functional recombinant progeny in the next generation. The recombination data in this study also provide a basis for an empirical test of this hypothesis.