No abstract

Hybridization between two chromosomally distinct subspecies of the grasshopper Caledia captiva results in a high incidence of novel chromosomal rearrangements among the backcross progeny. Rearrangements are restricted to those chromosomes derived from the F1 hybrid parent. Chromosomal involvement is nonrandom with the same rearrangement occurring repeatedly in different backcrosses. A single individual can also generate an array of different rearrangements among its offspring. Several of the rearrangements have also been found in natural populations. The nonrandom and recurrent nature of these chromosomal mutations at high frequencies provides a plausible explanation for the establishment and fixation of chromosomal rearrangements in natural populations.

A ribosomal RNA-encoding DNA (rDNA) cloned sequence, consisting of a 0.8-kilobase fragment from the 26S/nontranscribed spacer region, was used to identify diagnostic restriction enzyme fragments that distinguish the Moreton and Torresian subspecies of the grasshopper Caledia captiva. These restriction fragments were then used to study patterns of rDNA variation across a narrow geographical hybrid zone between the two subspecies. The pattern of rDNA variation that emerged after the analysis ofover 250 individuals clearly demonstrates the asymmetrical introgression of the Moreton ribosomal RNA genes into the Torresian subspecies. This asymmetric movement of genetic material occurs even though there exists extreme postmating F2 and backcross inviability between the two subspecies. From our data, as well as those of previous chromosomal and allozymic studies, we are able to support the occurrence of nonrandom processes such as biased gene conversion and/or natural selection. Because the rDNA loci in the Moreton and Torresian individuals are located in different regions on chromosomes 10 and 11, it should be possible to determine the relative contributions of conversion, natural selection, and these sorts of processes to the pattern of introgression of the Moreton rDNA into the Torresian subspecies.How reproductive isolation develops during the speciation process remains a fundamental and unanswered question in evolutionary biology. As a corollary ofthis question, one can inquire about the consequences of repeated hybridization between partially reproductively isolated populations. Grant (1) has stated, in reference to the findings of Anderson (2), that long-term backcrossing of hybrid individuals to the parental types can result in ".... convergences between previously separate phyletic lines ...." However, it is also apparent that the introduction, maintenance, and persistence of foreign genetic material, despite significant pre-and/or postmating isolation, must be mediated by factors such as genetic drift, migration, and/or natural selection. Thus, the definition of interacting processes that lead to introgression (2) of genetic material is of fundamental importance in understanding how genetic systems may be perturbed. Indeed, it has been advocated that a major stimulus to speciation involves hybridization (3). Furthermore, the quantitation of the effects of factors such as genetic drift, migration, and selection is equally fundamental in understanding the evolution of organisms.In order to examine the above parameters, it is necessary to identify hybridizing taxa that have the following characteristics: (i) significant levels of reproductive isolation; (ii) morphological, physiological, or genetic markers that can be used to identify genomic components in parental and hybrid individuals; and (iii) definable differences in the habitats occupied by the taxa. Such a system has been identified in the grasshopper Caledia captiva. In particular, two subspecies (Moreton and Torresian) of C. captiva meet and f...

The karyotypes of 9 species of parrot (Psittacidae), lorikeet (Loriidae) and cockatoo (Cacatuidae) are presented togemer with C-band data on 5 of the species. AU cockatoos possess a similar karyotype, which is very distinct from those observed in lorikeets and parrots. Even though many species are kept in captivity, few have been karyotyped. Those that have are 13 species from central and South America, 5 from the Afro-Asian region and 9 from Australasia, comprising three parrots and six cockatoos (summarized in VAN DONCEN and DE BOER 1984; SCHMUTZ and PRUS 1987). Despite the few species studied, the available data indicate considerable karyotypic variability within the order. To add to the karyotypic information for the order we describe here the karyotypes of three cockatoos, three parrots and two lorikeets from Australasia, together with that of one African parrot. C-banded karyotypes are also presented for five of these species. The patterns of chromosomal evolution in the Psittaciformes are summarized and discussed.

A hydrid 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 hydrid 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 hydridising taxa.