The heterochromatin of grasshoppers from the Caledia captiva species complex

C-band variation between the Caledia taxa is extensive with numerous large interstitial and telomeric blocks of heterochromatin being present in the South-east Australian and Moreton taxa while the Torresian types possess small centromeric or telomeric C-bands. In situ hybridization using ³H-cRNA from a 168 bp (base pairs) highly repeated sequence, originally isolated from the South-east Australian taxon, defined further variation between the C. captiva taxa. This sequence family is present in each of the interstitial and telomeric constitutive heterochromatic blocks in the South-east Australian and Moreton taxa. However, it is represented in only a fraction of the heterochromatic regions, defined by C-banding, within the three Torresian types. A second, unrelated 144 bp sequence family, originally isolated from the Daintree taxon, is restricted to the procentric blocks of heterochromatin of chromosomes 2–7, 9 and 10 in the Daintree taxon. This sequence is A-T rich and possesses a region of dyad symmetry. Quantitative measurements for the two sequence families revealed a wide range of copy numbers between the C. captiva taxa. The 168 bp family has approximately 150,000, 35,000 and 4,000 copies, respectively, in the South-east Australian/ Moreton, Torresian and Daintree genomes. There are 2,000,000 and 100,000 copies of the 144 bp sequence in the Daintree and Papuan Torresian taxa, respectively. The distributional, quantitative and sequence characteristics of these repeat families imply that past amplification or introgression has played a major role in the evolution of these sequences. There is an overall negative correlation between the quantity of the 168 bp sequence and the levels of reproductive isolation and genie divergence between the various taxa. It is possible that some of the reduction in the viability of the hybrid individuals is due to the quantitative changes in these sequences. Moreover, the quantitative and qualitative characteristics of highly repeated DNA families may play a role in the modulation of such essential cellular functions as cell cycle duration, nuclear organization and gene expression.     

The chromosomal component of reproductive isolation in the grasshopper Caledia captiva

An analysis of the relative viabilities of recombinant and nonrecombinant chromosomes among the surviving embryos from back-crosses involving the Moreton (M) and Torresian (T) taxa has revealed that these embryos do not contain a representative sample of gametes derived from the F₁ hybrid parent. The significant deviations in the hybrid gametic population arise entirely from intrachromosomal effects with no evidence of any between-chromosome interactions. This is interpreted as clear evidence to show that recombinational repatterning within heterozygous bivalents in the F₁ parent is a significant factor in inducing the observed deviant segregation ratios. Furthermore, by using a population which is chromosomally equivalent to the Torresian but genically similar to the Moreton, it has been shown that over 46% of the F₂ embryonic breakdown arises solely from the effects of chromosomal heterozygosity upon recombination repatterning among (Moreton × Torresian) F₁ hybrids. From these data it is proposed that each chromosome is internally coadapted in the sense that it contains balanced blocks of cis-acting acting loci which can be disrupted by recombinational change. Disruption of the linear association of the genes on structurally different chromosomes by recombination repatterning results in novel intrachromosomal associations which may be functionally inadequate and so lead to arrested embryonic development. It is speculated that an important factor in arresting development may involve interactions between the novel recombinant chromosomes of the gamete and maternal factors laid down in the egg during oogenesis which are responsible for the sequential activation of the genomes of the progeny during development. Thus coadaptation is interpreted in terms of the functional intergration of a chromosome with the products of the genome of the previous generation. The assessment of the relative viabilities of recombinant and nonrecombinant chromosomes has shown that the Torresian nonrecombinant chromosomes possess the highest viabilities in the sequence T^N>M^NT^R = M^R where N and R represent nonrecombinant and recombinant classes. This sequence is relevant to the structure of the hybrid zone between the Torresian and Moreton taxa and explains both its asymmetry and the basis of the observed introgression of Torresian chromosomes into the Moreton taxon and the absence of the reverse movement.     

The spatial distribution of chromosomes in metaphase neuroblast cells from subspecific F1 hybrids of the grasshopper Caledia captiva

The spatial distribution of chromosomes has been analysed in radial metaphase neuroblast cells in F₁ hybrid embryos generated by crossing individuals of the Moreton and Torresian (TT) chromosomal taxa of the grasshopper Caledia captiva. The Moreton individuals were of two kinds depending on whether they carried an acrocentric X (MAX) or a metacentric X (MMX). No significant associations were detected between any pair of homologous chromosomes in either male or female (MAX x TT) and (MMX x TT) F₁ hybrids. This result was supported by data which showed that the mean separation between homologues is greater, although not significantly so, than the mean separation between non-homologous chromosomes within the two Moreton genomes. Indeed, in a number of cases, genome separation was clearly observed in radial metaphase preparations from these F₁ hybrids. By comparison the analysis of pairwise associations between non-homologous chromosomes within the MMX and MAX Moreton genomes revealed a number of significant associations and dissociations which strongly suggests that at least some chromosomes in these genomes are organised non-randomly at metaphase. Of particular interest was the highly significant X-5 association in the MMX genome since in a previous study X-5 rearrangements were found to occur repeatedly among different backcross progeny involving Moreton x Torresian F₁ hybrids. Additionally a comparison of the organisation of chromosomes in the MAX and MMX genomes, which differ primarily by the type of X chromosome, revealed that in a number of cases pairs of chromosomes are arranged very differently with respect to each other. The distribution of chromosomes on the hollow spindle was also analysed to investigate whether a specific spatial ordering of chromosomes exists within these Moreton genomes based on the association of pairs of short arms and pairs of long arms of most similar length (the Bennett model). The twelve chromosomes in both genomes were uniquely ordered in a single chain. However, because of computing limitations, only the ordered arrangement of chromosomes 1–10 was investigated. An analysis of 48 cells in the MMX and 38 cells in the MAX genomes showed that the predicted order in the ten chromosome sub-set in each genome did not rank in the top 20% of the 181,440 possible orders. This suggests that, although there is a good evidence that some non-homologous chromosomes may be associated non-randomly at metaphase in these genomes, they do not appear to show a specific, ordered arrangement as predicted by the Bennett model. The significance of the observed non-random organisation of chromosomes in the MMX and MAX genomes is discussed in relation to the generation of novel chromosome rearrangements in Moreton x Torresian F₁ hybrids and the evolution of the Moreton and Torresian genomes.     

Developmental changes in the isoenzymes controlling glycolysis in the acridine grasshopper,Caledia captiva

Summary In this paper is reported an example of extensive developmental changes in the isoenzymes controlling a biochemical pathway: more than half of the glycolytic enzymes of the grasshopper,Caledia captiva differ in electrophoretic phenotype between embryonic and adult stages. A similar pattern of changes is found in each of the taxa ofC. captiva, which is actually a species complex. The present example of developmental variation differs from that described for glycolytic enzymes in vertebrates in two main points. Firstly, the changes between the phenotypes of the embryos and adults are co-incident in time, occurring near hatching. Secondly, in contrast to vertebrates where embryospecific isoenzymes are rare, there exist inC. captiva isoenzymes of trehalase, glucosephosphate isomerase, aldolase, pyruvate kinase, lactate dehydrogenase and 6-phosphogluconate dehydrogenase which are found in the embryo but not in the adult. Some of the variable enzymes also exhibit tissue specificity in the adult. The existence of the changes, whatever their basis, shows that the theory that the expression of housekeeping genes is developmentally invariant is not generally correct.     

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 adaptive role of facultative embryonic diapause in the grasshopper Caledia captiva (Orthoptera: Acrididae) in southeastern Australia

In a temperate zone population of the grasshopper Caledia captiva (Fabnerus) (Orthoptera Acrididae), facultative embryonic diapause provided the basis for a number of aspects of life cycle flexibility Most offspring produced during the summer entered diapause, destined to overwinter as embryos and hatch in the spring, but a small percentage developed directly to hatching, overwintered as nymphs and reproduced in the spring The two different developmental pathways often occurred among the offspring of the same mother, and could represent a bet-hedging adaptation Offspring of the small overwintering generation were influenced by a maternal effect which greatly diminished their likelihood of entering diapause This is an adaptation to prevent an inappropriate developmental delay that would negate the fitness advantage of an extra generation Offspring produced in the autumn failed to develop to the stage at which diapause intervenes before winter and overwintered at a pre-diapause stage They did not enter diapause when development resumed in the spring Facultative diapause in this case also prevented an inappropriate developmental delay in the spring, and provided females with the option of reproducing toward the end of the growing season Life cycle flexibility appears to have evolved to exploit a climate with a short growing season but mild winter     

The experimental verification of natural hybridization between two taxa of Hawaiian Bidens (Asteraceae)

Bidens ctenophylla andB. menziesii var.filiformis are two heterozygous, self-fertile but outcrossing, interfertile taxa. A relatively fertile intermediate population occurs on the “hybridized habitat” of a recent lava flow. This population contains some indi viduals which are similar toB. ctenophylla, some which are similar toB. menziesii var.filiformis, and many individuals of various intermediate forms. Variation studies of greenhouse populations of the two species, the F₁ hybrid, the F₂ hybrid, and the progeny of three wild intermediates confirm the field observation that the wild intermediate population is a product of natural hybridization betweenB. ctenophylla andB. menziesii var.filiformis. The analysis of two interspecific characters, stem anthocyanin and achene awns, indicates that each is polygenic and is regulated by relatively few genes.     

Environmental predictions and distributional limits of chromosomal taxa in the Australian grasshopperCaledia captiva (F.)

Summary Correlations between environmental parameters and the distribution of four chromosomal taxa in the acridine grasshopperCaledia captiva (F.) have been analyzed. A bioclimate prediction system (BIOCLIM) was used as a model to determine their bioclimatic envelopes and to predict their probable geographic distributions, based on climatic similarities with actual distributions. New, and formerly unsuspected, distribution areas are predicted. Parapatric boundaries are defined that actually coincide with known limits. The general distribution of the four taxa is correlated with rainfall seasonality and a temperature gradient. Their distributional boundaries are mostly in accordance with extreme values in rainfall in an east-west direction and temperature in a north-south direction. A nonmetric multidimensional scaling analysis of one selected narrow parapatric boundary confirms the nature of the geographic correlation with several climatic extreme values. The climatic and vegetation analyses suggest that sharp parapatric distributions can be maintained by gradual environmental influences and that coincidence with abrupt ecotones is not necessary. At least one of the parapatric boundaries seems to be relatively recent in origin (2000–6000 years) and on the basis of the location of extreme climatic values as well as chromosomal and genetic markers it is expected to slowly move further south and east of its present location apparently following a climatically established boundary.     

The heterochromatin of grasshoppers from the Caledia captiva species complex. I. Sequence evolution and conservation in a highly repeated DNA family

The restriction enzyme TaqI digests 0.2% of the genomic DNA from the grasshopper Caledia captiva to a family of sequences 168 bp in length (length of consensus sequence). The sequence variation of this "Taq family" of repeat units was examined among four races from C. captiva to assay the pattern of evolution within this highly repeated DNA. The Taq-family repeats are located in C-banded heterochromatin on at least one member of each homologous pair of chromosomes; the locations range from centromeric to telomeric. Thirty-nine cloned repeats isolated from two population 1A individuals along with 11 clones from seven populations taken from three of the races demonstrated sequence variation at 72 positions. Pairwise comparisons of the cloned repeats, both within an individual and between different races, indicate that levels of intraspecific divergence, as measured by reproductive incompatibility, do not correlate with sequence divergence among the 168-bp repeats. A number of subsequences within the repeat remain unchanged among all 50 clones; the longest of these is 18 bp. That the same 18-bp subsequence is present in all clones examined is a finding that departs significantly (P less than 0.01) from what would be expected to occur at random. Two other cloned repeats, from a reproductively isolated race of C. captiva, have sequences that show 56% identity with this 18-bp conserved region. An analysis showed that the frequency of occurrence of an RsaI recognition site within the 168- bp repeat in the entire Taq family agreed with that found in the cloned sequences. These data, along with a partial sequence for the entire Taq family obtained by sequencing uncloned repeats, suggest that the consensus sequence from the cloned copies is representative of this highly repeated family and is not a biased sample resulting from the cloning procedure. The 18-bp conserved sequence is part of a 42-bp sequence that possesses dyad symmetry typical of protein-binding sites. We speculate that this may be significant in the evolution of the Taq family of sequences.      

Evidence for association of chromosomal form and development time from complex clines and geographic races in the grasshopper Caledia captiva (Orthoptera: Acridadae)

Evidence for an association between chromosomal form and development time in the grasshopper Caledia captiva (F.) was obtained through comparison of two geographic taxa and analysis of a complex latitudinal cline within one of the taxa. Northern populations of the Moreton taxon possess a metacentric genome and are slow-developing. In contrast, the Torresian taxon, distributed throughout northern, coastal Australia, a region of pronounced seasonality in rainfall, and southern populations of the Moreton taxon, which inhabit a region of pronounced seasonality in temperature, Soth have an acrocentric genome and are fast-developing. The convergence of chromosomal form and development time between Torresian and southern Moreton populations appears to be driven by convergence in life history. Seasonality limits grasshoppers to one generation per year and favours fast development. The transition between relatively acrocentric southern Moreton populations and relatively metacentric northern Moreton populations is gradual but not monotonic. Instead, a shift to a bivoltine life history in the middle of the transect occurs and is associated with shifts in both development time and chromosomal form. These results imply an adaptive role for chromosomal form, although the causative link between chromosomal variation and variation in development time remains to be established.     

Sexually differentiated mechanisms of sterility in interspecific hybrids between Oryzias latipes and O. curvinotus.

Fertility of interspecific hybrids between Oryzias latipes and O. curvinotus was examined. F1 females were able to lay eggs but males were sterile. Histological examination of the ovaries of hybrids revealed that oogenesis does not proceed normally in spite of the apparent fertility. Most oocytes degenerated at the pachytene stage of the meiotic prophase, and only a few entered the diplotene stage to develop into ova. Hybrid males could induce females to spawn eggs, an indication that they had differentiated completely into true males. However, they did not produce fertile sperm. Most germ cells in testes of hybrids passes through almost the entire process of spermatogenesis, but deviations from the normal course of events were observed during spermiogenesis. The condensation of chromatin in spermatids occurred, but the diameters of sperm heads were about 1.5-fold larger than those of normal ones. Prominent abnormalities were apparent in the quantity and arrangement of microtubules in the cytoplasm. Abnormal spermatozoa were phagocytized by Sertoli cells. These observations indicate that the mechanisms of impaired gametogenesis in these interspecific hybrids are sexually differentiated.     

Genome size and base composition variation in natural and experimental Narcissus (Amaryllidaceae) hybrids

Background and Aims

Although there is evidence that both allopolyploid and homoploid hybridization lead to rapid genomic changes, much less is known about hybrids from parents with different basic numbers without further chromosome doubling. Two natural hybrids, Narcissus × alentejanus (2n = 19) and N. × perezlarae (2n = 29), originated by one progenitor (N. cavanillesii, 2n = 28) and two others (N. serotinus, 2n = 10 and N. miniatus, 2n = 30, respectively) allow us to study how DNA content and composition varies in such hybrids.

Methods

Flow cytometry measurements with two staining techniques, PI and DAPI, were used to estimate 2C values and base composition (AT/GC ratio) in 390 samples from 54 wild populations of the two natural hybrids and their parental species. In addition, 20 synthetic F₁ hybrid individuals were also studied for comparison.

Key Results

Natural hybrids presented 2C values intermediate between those found in their parental species, although intra-population variance was very high in both hybrids, particularly for PI. Genome size estimated from DAPI was higher in synthetic hybrids than in hybrids from natural populations. In addition, differences for PI 2C values were detected between synthetic reciprocal crosses, attributable to maternal effects, as well as between natural hybrids and those synthetic F₁ hybrids in which N. cavanillesii acted as a mother.

Conclusions

Our results suggest that natural hybrid populations are composed of a mixture of markedly different hybrid genotypes produced either by structural chromosome changes, consistent with classic cytogenetic studies in Narcissus, or by transposon-mediated events.     

Experimental trypanosomiasis of natural hybrids between house mouse subspecies.

This study characterises the extent of the susceptibility to parasites (first demonstrated with helminths) of hybrids between Mus musculus domesticus and Mus musculus musculus. Experimental infections with Trypanosoma musculi of M. m. domesticus, M. m. musculus and their natural hybrids have been performed to compare their level of resistance/susceptibility. It appears that contrary to the results with helminths, hybrid mice present the same level of resistance/susceptibility to the trypanosome as M. m. musculus and M. m. domesticus individuals. This result is interpreted in the light of the modalities of host parasite interactions and leads us to hypothesise on the role of parasitism in the evolution of the house mouse hybrid zone.     

Establishment and characterization of somatic hybrids between human differentiated macrophages and mouse myeloma NS1 cells

Human macrophages obtained from circulating monocytes matured in vitro by culture for seven days in hydrophobic flexible teflon bags were successfully fused with murine myeloma NS1 cells. Six of 20 clones, selected for their adherence properties, were further studied. All possessed human chromosomes (mean number ranging from 4 to 14 depending on the clones studied), exhibited non-specific esterases (but no peroxidase activity) and expressed CD14 antigen and C3 receptors (but no MAX-1 antigen). Moreover, the hybridomas retained phagocytic activity and high interferon plus lipopolysaccharide-activable cytolytic activity against tumor cells.     

Susceptibility of natural hybrids between house mouse subspecies to Sarcocystis muris

A previous study showed that the susceptibility of hybrids between two Mus musculus musculus and Mus musculus domesticus did not apply to every parasite. The authors proposed that only parasites which exerted enough constraints would induce the selection of poorly compatible systems of resistance in the subspecies. This study completes the previous work. Experimental infections of mice of the two subspecies and their hybrids with the tissue-dwelling protozoan Sarcocystis muris show that hybrids are more susceptible to the coccidian than M. m. musculus and M. m. domesticus individuals. This result demonstrates that the hybrids are not only susceptible to intestinal helminths, and confirms the 'constraint hypothesis'.     

The distribution and inversion polymorphism of chromosomally recognized taxa of the Anopheles gambiae complex in Mali, West Africa

Data from polytene chromosome studies on the Anopheles gambiae complex in Mali were reviewed. The banding pattern was successfully scored in 17,705 specimens from 76 sampling sites representing the main ecological strata of the country. Two members of the complex, namely An. arabiensis and An. gambiae, were found widespread and frequently sympatric, with the latter prevalent in most localities. Population genetic analysis of the inversion polymorphisms indicated the existence of panmictic conditions for An. arabiensis only, whereas the parallel study of An. gambiae supported its splitting into at least three reproductive units, characterized by different 2R chromosome arrangements, designated Bamako, Mopti and Savanna. The chromosomal evidence was consistent with the hypothesis of complete reproductive isolation between Bamako and Mopti. Partial isolation between these two taxa and Savanna was suggested by the scoring of hypothetical hybrid 2R heterokaryotypes in various samples, but the actual hybrid origin of these specimens was not confirmed. Different patterns of geographical and seasonal distribution were shown as follows. An. arabiensis prevails in arid savannas (Sahel and Northern Sudan savanna) out of the flooded or irrigated zones; it is able to withstand the most arid conditions of Saharan localities and its breeding might extend throughout the dry season. An. gambiae Savanna and Bamako prevail in relatively humid savannas (Southern Sudan savanna) and their breeding generally occurs only during the rainy season. The Savanna taxon was almost absent in flooded or irrigated zones and in riverine localities; the Bamako taxon is distributed along the upper river Niger and its tributaries. An. gambiae Mopti extends its range in all ecological zones present in Mali including the Sahel and predesertic areas, showing high relative frequencies up to absolute dominance in flooded or irrigated areas; its breeding is highly successful also during the dry season. Rainfall at the sampling sites was found to correlate positively with the frequency of Savanna and negatively with the frequency of Mopti. The remarkable ecological flexibility of the latter was found associated with wide seasonal and geographical variations in its 2R inversion polymorphism bc/u. Higher frequencies of the bc arrangement were recorded both in the Southern localities during the dry season and in the Northern more arid localities during the rainy season. The absence or scarcity of An. arabiensis and An. gambiae Savanna in most flooded or irrigated zones suggests their competitive exclusion by An. gambiae Mopti.     

Flow cytometric analysis and microsatellite genotyping reveal extensive DNA content variation in Trypanosoma cruzi populations and expose contrasts between natural and experimental hybrids

Trypanosoma cruzi exhibits remarkable genetic heterogeneity. This is evident at the nucleotide level but also structurally, in the form of karyotypic variation and DNA content differences between strains. Although natural populations of T. cruzi are predominantly clonal, hybrid lineages (TcIId and TcIIe) have been identified and hybridisation has been demonstrated in vitro, raising the possibility that genetic exchange may continue to shape the evolution of this pathogen. The mechanism of genetic exchange identified in the laboratory is unusual, apparently involving fusion of diploid parents followed by genome erosion. We investigated DNA content diversity in natural populations of T. cruzi in the context of its genetic subdivisions by using flow cytometric analysis and multilocus microsatellite genotyping to determine the relative DNA content and estimate the ploidy of 54 cloned isolates. The maximum difference observed was 47.5% between strain Tu18 cl2 (TcIIb) and strain C8 cl1 (TcI), which we estimated to be equivalent to 73 Mb of DNA. Large DNA content differences were identified within and between discrete typing units (DTUs). In particular, the mean DNA content of TcI strains was significantly less than that for TcII strains (P < 0.001). Comparisons of hybrid DTUs TcIId/IIe with corresponding parental DTUs TcIIb/IIc indicated that natural hybrids are predominantly diploid. We also measured the relative DNA content of six in vitro-generated TcI hybrid clones and their parents. In contrast to TcIId/IIe hybrid strains these experimental hybrids comprised populations of sub-tetraploid organisms with mean DNA contents 1.65–1.72 times higher than the parental organisms. The DNA contents of both parents and hybrids were shown to be relatively stable after passage through a mammalian host, heat shock or nutritional stress. The results are discussed in the context of hybridisation mechanisms in both natural and in vitro settings.     

Expression of differentiated properties in fetal liver cells and their somatic cell hybrids

A line of murine fetal liver cells is described and characterized. Hybrids between these differentiated cells and a non-differentiated fibroblast line were isolated and studied for the expression of the differentiated characteristics of the parent liver line. These include expression and inducibility for the enzyme tryptophan pyrrolase, ability to accumulate glycogen granules, and a characteristic morphology and growth rate, all of which were suppressed in the hybrid. The results obtained from sequential chromosome counts of the hybrid indicate that the modal number drops from about 100 to about 72 in 5 months, a fact that may be related to the extreme differences in generation times of the two parents.     

Fitness of hybrids between rapeseed (Brassica napus) and wild Brassica rapa in natural habitats

Fitness of hybrids between genetically modified (GM) crops and wild relatives influences the likelihood of ecological harm. We measured fitness components in spontaneous (non-GM) rapeseed x Brassica rapa hybrids in natural populations. The F1 hybrids yielded 46.9% seed output of B. rapa, were 16.9% as effective as males on B. rapa and exhibited increased self-pollination. Assuming 100% GM rapeseed cultivation, we conservatively predict < 7000 second-generation transgenic hybrids annually in the United Kingdom (i.e. approximately 20% of F1 hybrids). Conversely, whilst reduced hybrid fitness improves feasibility of bio-containment, stage projection matrices suggests broad scope for some transgenes to offset this effect by enhancing fitness.