Putative Parental Species Research Articles

Invasive Mahonia plants outgrow their native relatives

Invasive populations often grow more vigorously than conspecific populations in the native range. This has frequently been attributed to evolutionary changes resulting either from founder effects, or from natural selection owing to enemy release. Another mechanism contributing to evolutionary change has largely been neglected in the past: Many invasive plant species do actually descend from cultivated plants and were therefore subject to breeding, including hybridization and artificial selection. In a common garden experiment, we compared invasive Central European populations of the ornamental shrub, Mahonia, with native populations of its putative parental species, Mahonia aquifolium and M. repens, from North America. We hypothesized that plants of invasive populations show increased growth and retained high levels of heritable variation in phenotypic traits. Indeed, invasive Mahonia plants grew larger in terms of stem length, number of leaves and above-ground biomass than either of the two native species, which did not differ significantly from each other. Since there are no hints on release of invasive Mahonia populations from natural enemies, it is likely that hybridization and subsequent selection by breeders have lead to an evolutionary increase of plant vigour in the introduced range. Further on, heritable variation was not consistently reduced in invasive populations compared with populations of the two native species. We suggest that interspecific hybridization among the Mahonia species has counteracted the harmful effects of genetic bottlenecks often associated with species introductions. Based on this case study, we conclude that, more attention has to be paid on the role of plant breeding when assessing the mechanisms behind successful plant invasions in future.

Molecular evidence for double maternal origins of the diploid hybrid Hippophae goniocarpa (Elaeagnaceae)

Homoploid hybrid plant species are rare, and the mechanisms of their speciation are largely unknown, especially for homoploid hybrid tree species. Two contrasting hypotheses have been proposed to explain the origin of Hippophae goniocarpa: (1) it is a diploid hybrid originating from H. rhamnoides ssp. sinensis x H. neurocarpa ssp. neurocarpa, and (2) it originated via marginal differentiation from H. rhamnoides ssp. sinensis. Regardless of which of these hypotheses is true (if either), previous studies have suggested that H. rhamnoides ssp. sinensis is the only maternal donor for this hybrid species. In this study, we aim to elucidate the maternal composition of H. goniocarpa and to test the two hypotheses. For this purpose, we sequenced the maternal chloroplast DNA trnL-F region of 75 individuals representing H. goniocarpa, H. rhamnoides ssp. sinensis, and H. neurocarpa ssp. neurocarpa in two co-occurring sites of the taxa. Seven haplotypes were identified from three taxonomic units, and their phylogenetic relationships were further constructed by means of maximum parsimony, maximum likelihood, and network analyses. These seven haplotypes clustered into two distinct, highly divergent lineages. Two haplotypes from one lineage were found in H. rhamnoides ssp. sinensis, and five (representing the other lineage) in H. neurocarpa ssp. neurocarpa. Hippophae goniocarpa shared four common haplotypes from both lineages, but the haplotypes detected from the two populations differed to some extent, and in each case were identical to local haplotypes of the putative parental species. Thus, both H. rhamnoides ssp. sinensis and H. neurocarpa ssp. neurocarpa appear to have together contributed to the maternal establishment of H. goniocarpa. These results clearly demonstrate that the marginal origin hypothesis should be rejected, and support the hybrid origin hypothesis. Hippophae goniocarpa exhibits a sympatric distribution with its two parent species, without occupying new niches or displaying complete ecological isolation. However, this species has effectively developed reproductive isolation from its sympatric parent species. Our preliminary results suggest that H. goniocarpa may provide a useful model system for studying diploid hybrid speciation in trees. (c) 2008 The Linnean Society of London.

Gene flow and the genealogical history of Heliconius heurippa

BackgroundThe neotropical butterfly Heliconius heurippa has a hybrid colour pattern, which also contributes to reproductive isolation, making it a likely example of hybrid speciation. Here we used phylogenetic and coalescent-based analyses of multilocus sequence data to investigate the origin of H. heurippa.ResultsWe sequenced a mitochondrial region (CoI and CoII), a sex-linked locus (Tpi) and two autosomal loci (w and sd) from H. heurippa and the putative parental species, H. cydno and H. melpomene. These were analysed in combination with data from two previously sequenced autosomal loci, Dll and Inv. H. heurippa was monophyletic at mtDNA and Tpi, but showed a shared distribution of alleles derived from both parental lineages at all four autosomal loci. Estimates of genetic differentiation showed that H. heurippa is closer to H. cydno at mtDNA and three autosomal loci, intermediate at Tpi, and closer to H. melpomene at Dll. Using coalescent simulations with the Isolation-Migration model (IM), we attempted to establish the incidence of gene flow in the origin of H. heurippa. This analysis suggested that ongoing introgression is frequent between all three species and variable in extent between loci.ConclusionIntrogression, which is a necessary precursor of hybrid speciation, seems to have also blurred the coalescent history of these species. The origin of Heliconius heurippa may have been restricted to introgression of few colour pattern genes from H. melpomene into the H. cydno genome, with little evidence of genomic mosaicism.

Hybridization and morphogenetic variation in the invasive alien Fallopia (Polygonaceae) complex in Belgium

The invasive alien knotweeds, Fallopia spp. (Polygonaceae), are some of the most troublesome invasive species in Europe and North America. Invasive success in Fallopia may be enhanced by multiple hybridization events. We examined the pattern of hybridization and its evolutionary consequences in Belgium with a concerted analysis of ploidy levels (chromosome counts and flow cytometry), morphological variation, and genetic variation (RAPDs). At least four taxa with different ploidy levels were part of the pattern of invasion in Belgium. Hybrid F. ×bohemica with various chromosome numbers restored the genotypic diversity that was lacking in the parental species. Hybrid genotypes were mainly assigned to a specific genetic pool and not to a mixture between the genetic pools of the putative parental species as would be expected for hybrids. Parental species and hexaploid hybrids differed significantly for a set of well-defined morphological characters, enabling future researchers to distinguish these taxa. On the basis of our results, the importance of hybridization has probably been underestimated in large parts of the adventive range of alien Fallopia species, pointing to the need for concerted molecular and morphological analyses in the study of the evolutionary consequences of hybridization.

Hybridization of reef fishes at the Indo-Pacific biogeographic barrier: a case study

Hybridization is recognized as an important source of genetic variation. In some reef fishes, including the Acanthuridae, hybridization has been detected due to intermediate colouration. This study used a molecular genetic approach to investigate hybridization in two Acanthurid species: Acanthurus leucosternon and Acanthurus nigricans, which have Indian and Pacific Ocean distributions respectively and are sympatric in the eastern Indian Ocean. In this area a putatitve hybrid, Acanthurus cf. leucosternon has been recognized based on intermediate colouration and restriction to the sympatric region of otherwise allopatric putative parental species. This study aimed to test this hypothesis using genetic tools. The three species were sampled from Cocos (Keeling) and Christmas Islands, the biogeographic boundary where many Indian and Pacific Ocean biota meet. Representatives from allopatric populations of both parental species and outgroups were also sampled. Mitochondrial COI and intron 1 of the nuclear ribosomal protein S7 were sequenced from 13 and 30 specimens respectively. Although sample sizes in this study are relatively small and more genetic data, including an extended phylogeographic sampling, is required to further evaluate these findings, the COI results support hybrid origins of Acanthurus cf. leucosternon, but S7 data are inconclusive due to the possibility of incomplete lineage sorting. The fourfold more abundant Acanthurus nigricans is most often the maternal parent. Inter-fertile hybrids apparently backcross with rare Acanthurus leucosternon males, transferring Acanthurus nigricans mitochondria to this species. These results suggest that Acanthurus leucosternon may eventually be lost from these islands, due to their relative rarity and introgressive hybridization.

ITS sequence variation supports the hybrid origin ofMalus toringoidesHughes

Malus toringoides (Rehd.) Hughes was suggested to have originated from hybridization between Malus transitoria Schneid. and Malus kansuensis Rehd., followed by repeated backcrossing to one of the putative parents. In the present study, the sequence information of the internal transcribed spacer (ITS) of nuclear ribosomal DNA (nrDNA) was used to re-examine the origin of this species. A total of 69 accessions from three natural populations (Maerkang, Xiaba and Kehe, Aba Autonomous Region, Sichuan, China) of M. toringoides and 10 accessions of its putative parents were analyzed. Using Malus angustifolia (Ait.) Michx., Malus ioensis (Wood) Britt. and Malus doumeri Chev. as outgroups, our phylogenetic analysis of the ITS sequences of M. toringoides and its putative parents showed that M. toringoides was not monophyletic, and two different types of ITS sequences which were obtained from each of the six accessions of M. toringoides were found to have clustered separately with those of the two putative parent species on the gene tree. A comparison of the sequence variation between M. toringoides and its putative parents revealed an additive variation pattern of ITS sequences in the putative hybrid species. These results are consistent with the previous morphological and amplified fragment length polymorphism (AFLP) data which suggested that M. toringoides was of hybrid origin. Our ITS data provide new molecular evidence for the hybrid origin hypothesis of M. toringoides and these results are of great importance for future study on hybridization, polyploid speciation and evolution of the genus Malus Miller.

A rare hybridization event in two common Caribbean wrasses (genus Halichoeres; family Labridae)

Molecular tools were used to evaluate the hybrid status of a specimen with intermediate colour pattern between Halichoeres bivittatus and Halichoeres garnoti from Belize. Phylogenetic analyses of the two species, eight Halichoeres species from new and old world lineages and two outgroups showed that the study species are closely related and that H. garnoti is the maternal contributor to the putative hybrid specimen, based on partial mitochondrial COI data. Direct sequencing of Intron 1 of the nuclear ribosomal protein S7 identified H. bivittatus as sister to H. garnoti with the putative hybrid specimen in an intermediate position, due to heterozygosity at nucleotides alternatively fixed in the two putative parent species. This is consistent with the hybrid status of the specimen, with parental contributions from both H. garnoti and H. bivittatus. These results, combined with no evidence of introgression between the two parent species (based on the mtDNA and the single investigated nuclear marker) and the biogeography and ecology of these species suggests that this is a rare event with minimal evolutionary implications.

No genomic mosaicism in a putative hybrid butterfly species

Recent descriptions of hybrid animal species have spurred interest in this phenomenon, but little genomic data exist to support it. Here, we use frequency variation for 657 amplified fragment length polymorphism (AFLP) markers and DNA sequence variation from 16 genes to determine whether the genome of Heliconius pachinus, a suspected hybrid butterfly species, is a mixture of the putative parental species, Heliconius cydno and Heliconius melpomene. Despite substantial shared genetic variation among all three species, we show that the genome of H. pachinus is not a mosaic; both AFLP and DNA sequence data overwhelmingly associate H. pachinus with just one of the potential parents, H. cydno. This pattern also applies to the gene wingless, which is tightly linked to the locus that determines forewing colour-one specific H. pachinus trait that has been hypothesized to have originated from H. melpomene. As a whole, the data support a traditional, bifurcating model of speciation in which H. pachinus split from a common ancestor with H. cydno without a genetic contribution from H. melpomene. However, comparison of our data to DNA sequence data for another putative hybrid Heliconius species, Heliconius heurippa, suggests that the H. heurippa genome may be a mosaic.

Morphological and ploidy level variation ofCentaurea phrygia agg. (Asteraceae) in the Czech Republic, Slovakia and Ukraine

Centaurea phrygia agg. was studied in part of Central Europe (Czech Republic, Slovakia and Ukraine) to evaluate its karyological and morphological variation. Three ploidy levels were found: diploid (C. pseudophrygia, C. stenolepis and several populations ofC. phrygia s.str.), tetraploid (C. oxylepis and several populations ofC. phrygia s.str.), and triploid (very rare, only single individuals). Only one taxon,C. phrygia s.str., comprises populations of two ploidy levels, but they seem to be geographically separated. Multivariate morphometric analysis confirmed the separation of the four taxa; the length and the width of appendages of middle involucral bracts, visibility of appendages of inner involucral bracts, the length and the width of an involucre, and the length/width ratio of middle cauline leaves are the most important discriminant characters. Several populations intermediate between two species (eitherC. pseudophrygia andC. stenolepis orC. oxylepis andC. phrygia s.str.) were found. Their possible origin from introgressive hybridization is discussed considering their morphology (intermediate between putative parental species), chromosome numbers (always the same as in putative parents), and distribution (both spatially and ecologically more or less intermediate between distribution ranges of putative parents). A determination key for the four studied species, including intermediates, is also presented.

A NOVEL PREFERENCE FOR AN INVASIVE PLANT AS A MECHANISM FOR ANIMAL HYBRID SPECIATION

Homoploid hybrid speciation--speciation via hybridization without a change in chromosome number--is rarely documented and poorly understood in animals. In particular, the mechanisms by which animal homoploid hybrid species become ecologically and reproductively isolated from their parents are hypothetical and remain largely untested by experiments. For the many host-specific parasites that mate on their host, choosing the right host is the most important ecological and reproductive barrier between these species. One example of a host-specific parasite is the Lonicera fly, a population of tephritid fruit flies that evolved within the last 250 years likely by hybridization between two native Rhagoletis species following a host shift to invasive honeysuckle. We studied the host preference of the Lonicera fly and its putative parent species in laboratory experiments. The Lonicera fly prefers its new host, introduced honeysuckle, over the hosts of both parental species, demonstrating the rapid acquisition of preference for a new host as a means of behavioral isolation from the parent species. The parent taxa discriminate against each other's native hosts, but both accept honeysuckle fruit, leaving the potential for asymmetric gene flow from the parent species. Importantly, this pattern allows us to formulate hypotheses about the initial formation of the Lonicera fly. As mating partners from the two parent taxa are more likely to meet on invasive honeysuckle than on their respective native hosts, independent acceptance of honeysuckle by both parents likely preceded hybridization. We propose that invasive honeysuckle served as a catalyst for the local breakdown of reproductive isolation between the native parent species, a novel consequence of the introduction of an exotic weed. We describe behavioral mechanisms that explain the initial hybridization and subsequent reproductive isolation of the hybrid Lonicera fly. These results provide experimental support for a combination of host shift and hybridization as a model for hybrid speciation in parasitic animals.

Comparative analysis of micromorphological characters in two distantly related Mexican oaks, Quercus conzattii and Q. eduardii (Fagaceae), and their hybrids

Abstract Interspecific hybridization occurs with high frequency in the genus Quercus, but few studies have analyzed and compared micromorphological characters in putative parental species and their hybrids. Quercus eduardii and Q. conzattii are two Mexican black oak species that, although distantly related, have formed at least one population of hybrid origin, where individuals with intermediate macromorphology are present. The purpose of this investigation was to analyze the degree of differentiation in micromorphological characters between the two species and to assess the expression of these characters in individuals with intermediate macromorphology. Foliar trichomes, epicuticular waxes, stomata, and pollen grains, were examined using scanning electron microscopy (SEM) in the three types of individuals (Q. conzattii, Q. eduardii, and intermediates). Trichome density was quantified with light microscopy. Types of trichomes present, length of trichome arms, types of epicuticular wax on the leaf surfaces...

Chemistry of the Australian Gymnosperms. Part IX. The Leaf Oils of the Australian Members of the Genus Callitris (Cupressaceae)

The leaf oils of the 18 species and four subspecies of the genus Callitris endemic to Australia have been investigated by a combination of GC and GC/MS. All taxa produced oils in poor to moderate yields. Callitris baileyi produced a leaf oil in which α-pinene and limonene together, contributed the majority of the oil, while in Callitris canescens methyl citronellate also made significant contributions to the oil. In C. columellaris, limonene was the principal component, contributing up to 78% of the oil. Callitris drummondii gave a leaf oil which showed two chemical varieties. The samples from Western Australia contained large amounts of α-pinene (67–69%), while the sample from South Australia contained limonene (10.9%), bornyl acetate (24.2%) and geranyl acetate (14.9%) as significant components. In C. endlicheri the main components were limonene, α-pinene and bornyl acetate. Limonene and α-pinene were the principal components of the leaf oil of C. glaucophylla. Callitris gracilis ssp. gracilis gave an oil in which the principal components were α-pinene, myrcene and limonene, while in ssp. murrayensis a-pinene was the principal component. α-Pinene and limonene were the principal components of C. intratropica and C. macleayana. Callitris muelleri was found in two chemical forms, one monoterpenoid and one sesquiterpenoid, with either a-pinene or spathulenol being the principal component. In C. oblonga, which consists of three subspecies, α-pinene was the main component. In C. preissii the major components were α-pinene, myrcene, limonene and bornyl acetate. Callitris rhomboidea was distinguished from the vast majority of other Callitris species by containing significant amounts of neryl acetate, geranyl acetate and citronellyl acetate. Callitris roei contained significant amounts of sesquiterpenes in its leaf oil, with (E)-nerolidol being the principal component. In C. tuberculata α-pinene and limonene were the major components. α-Pinene, limonene and camphor were the major components in the leaf oil of C. verrucosa, while in Callitris sp. (Emerald Falls PI. Forster +PIF26357) α-pinene and limonene were the principal components with significant amounts of α-fenchyl acetate. The leaf oil of the putative introgressed populations of C. columellaris—C. verrucosa had α-pinene (25–46%), myrcene (9–19%), limonene (14–24%), α-fenchyl acetate (8–13%) and bornyl acetate (4–13%) as the most significant components. The oil does provide similarities to the oils of both putative parent species in that it contains camphor (1–3%), which is characteristic of C. verrucosa and bornyl acetate (2–13%) characteristic of C. glaucophylla.

Phenetic analyses ofHomoranthus(Myrtaceae: Chamelaucieae) on the basis of morphology

Homoranthus A.Cunn. ex Schauer is an endemic Australian genus widespread in southern and eastern mainland Australia. Phenetic analyses using clustering and ordination techniques were used to test the limits of all 23 currently recognised species. Data were analysed for 48 morphological characters from 139 specimens representing all species across their geographic ranges. Specimens of Homoranthus bornhardtiensis J.T.Hunter and H. prolixus Craven & S.R.Jones formed one group and appear to be the same entity. A putative natural hybrid between H. zeteticorum Craven & S.R.Jones and H. coracinus A.R.Bean was found to be phenetically intermediate between its putative parent species. Within each of H. biflorus Craven & S.R.Jones s.l., H. decumbens (Byrnes) Craven & S.R.Jones s.l., H. melanostictus Craven & S.R.Jones s.l. and H. porteri (C.T.White) Craven & S.R.Jones s.l., there are two or three distinct entities worthy of recognition as species. There was also strong support for the recognition of an entity from near Tenterfield in northern New South Wales as a distinct species most similar to H. montanus Craven & S.R.Jones. We conclude that a total of 28 species should be recognised within Homoranthus, including six new, currently undescribed species.

Homoploid Hybrid Speciation in an Extreme Habitat

According to theory, homoploid hybrid speciation, which is hybrid speciation without a change in chromosome number, is facilitated by adaptation to a novel or extreme habitat. Using molecular and ecological data, we found that the alpine-adapted butterflies in the genus Lycaeides are the product of hybrid speciation. The alpine populations possess a mosaic genome derived from both L. melissa and L. idas and are differentiated from and younger than their putative parental species. As predicted, adaptive traits may allow for persistence in the environmentally extreme alpine habitat and reproductively isolate these populations from their parental species.

Germination characteristics of native Japanese dandelion autopolyploids and their putative diploid parent species

Germination characteristics of native Japanese Taraxacum lineages of Taraxacum platycarpum (diploid), T. venustum (triploid and tetraploid), and T. albidum (pentaploid) have been studied at different temperatures. Taraxacum platycarpum ssp. hondoense is the putative diploid parent of T. venustum. Diploid T. platycarpum ssp. hondoense and the polyploids T. venustum and T. albidum are found in different areas of Japan, and distribution differences may reflect divergent ecological and physiological traits among ploidy levels. In this study, to prevent mixing of seeds of different polyploidy we used flow cytometry to examine the ploidy level of the plants from which seeds were collected. Results from seed-germination experiments showed that dependence on temperature of final percentage germination was qualitatively similar for both autopolyploids T. venustum and diploids T. platycarpum--germination was suppressed at high and low temperatures. It was also shown that seed germination of autopolyploids was suppressed more than that of the ancestral diploid at low temperatures and that seed germination for polyploids was higher than for the diploid. Threshold variations at low temperatures might affect the distribution of native dandelions. Taraxacum venustum, which occurs in cool climates, might have developed a distinctly lower germination threshold at low temperatures whereas T. albidum, which is native to warm climates, might have developed an adaptive threshold at high temperatures.

Usefulness of SSR Derived from Tetraploid Gossypium spp. for Analyses of Diploid Gossypium spp.

This investigation is the first attempt to utilize tetraploid species-derived primers to generate markers for mapping in diploid cotton species. About 113 individual F2 plants, developed from the cross between Gossypium nelsoni and G. australe, were used to construct a linkage map. One allotetraploid (ADj) G. hirsutum (TM-1) plant and its putative diploid ancestors, G. arboreum (A2) and G. raimondii (D5), were also included for comparative analysis. We developed a high-throughput multi-mix PCR assay for simple sequence repeat (SSR) marker separation using fluorescent-labeled capillary electrophoresis. Of a total of 205 tetraploid-derived SSR primers, 83% amplified in all four diploid species, indicating that flanking primer sequences are conserved in the diploid and tetraploid ADj genome of cotton. DNA sequence information of 18 randomly selected clones revealed that all of the clones contained SSR repeat motifs in the diploid species, suggesting that SSR markers including the flanking primer sequences have been conserved at the inter-specific level in genomic DNA across years of evolution. About 85% of the amplified primers showed polymorphism among the diploid genome species. Forty percent of SSRs in the diploid G. nelsoni and 35% in G. australe amplified the same size DNA fragment as in the tetraploid, (TM-1). This is further evidence that many of the tetraploid SSRs are conserved across diploid and tetraploid species. G. nelsoni compared with G. australe had more conserved genomic regions in common with G. hirsutum. From 73 markers scored in the F2 population, 50 exhibited codominance and 23 dominance. There were 47 markers linked across 10 linkage groups covering a total map distance of 754 cM with an average of 16 cM per marker. One linkage group (lg-3) with six loci has been assigned to chromosome 20. Comparative analysis of DNA sequences between G. hirsutum and its two putative parental diploid species indicated that changes within a few selected clones have occurred during evolution as follows: (1) by increasing or decreasing the number of the repeat motif, (2) by single nucleotide mutations occurring within or outside of the repeat motif, (3) by inter conversion of a simple to a compound SSR motif, and (4) by inversion of the repeat regions. Tetraploid-derived microsatellites should be useful in comparative genetic mapping of both tetraploid and diploid Gossypium species for evolutionary studies and introgression of agriculturally important traits from exotic diploid and tetraploid germplasm sources.

THE EVOLUTION OF CALIFORNIA'S WILD RADISH HAS RESULTED IN THE EXTINCTION OF ITS PROGENITORS

If two previously isolated taxa mutually assimilate through hybridization and subsequent biparental introgression, and if their introgressed descendants have the same or higher fitness than their parents, then gene flow should result in the local extinction of parental taxa via replacement by hybrid derivatives. These dramatic events may occur rapidly, even in a few generations. Given the speed at which such extinction by hybridization may occur, it may be difficult to identify that the process has occurred. Thus, documented instances of extinction by hybridization are rare, and especially so for cases in which both parents are replaced by the hybrid lineage. Here we report morphological and allozyme evidence for the local extinction of two Raphanus species in California via replacement by their hybrid-derived descendants. The results from a greenhouse experiment demonstrate that California wild radishes have a specific combination of traits from their progenitors, and comparison of our results to that of an earlier report indicate that pure parental types are no longer present in the wild. Our results also show the hybrid-derived lineage has transgressive fruit weight compared to its parents. Allozyme analysis demonstrates that California wild radishes are derived from hybridization between the putative parental species. However, that analysis also demonstrates that California wild radish has now become an evolutionary entity separate from both of its parents. We suggest that the aggressive colonizing behavior of the hybrid-derived lineage probably results from a novel combination of parental traits, rather than genetic variability of the population per se.

Hybridization in coral reef fishes: Introgression and bi-directional gene exchange in Thalassoma (family Labridae)

Hybrids in coral reef fishes have traditionally been described based on external features using meristic characters and colouration to identify putative parental contributors. This study utilised molecular genetic techniques to verify hybrid status and identify putative parental species for five hybrid specimens (Labridae: Thalassoma) collected from Holmes Reef in the Coral Sea. Phylogenetic analyses support hybrid origins of the specimens. Mitochondrial COI gene, nuclear S7 (intron 1) and nuclear copy of mitochondrial (NUMT) D-loop region corroborate the identity of T. quinquevittatum as the maternal and T. jansenii as the paternal contributor. Backcrossing to parental species by hybrids and bi-directional gene exchange between the Holmes Reef populations of T. jansenii and T. quinquevittatum was detected, suggesting that hybrids are fertile and able to reproduce successfully. F 1 hybrids display a mixture of the colouration attributes of the two parental species, but subsequent backcrossed individuals were unrecognisable as hybrids and displayed colouration of either parental species. A large numerical imbalance exists between the putative parental species at Holmes Reef, with T. quinquevittatum outnumbering T. jansenii by approximately 25:1. In this case study, hybridization appears to be driven by ecological rather than evolutionary factors.

Cytological evidence of natural hybridization in Brachiaria brizantha Stapf (Gramineae)

Two accessions of Brachiaria brizantha under cytological analysis showed 2n = 5x = 45 chromosomes. Pentaploidy probably resulted from natural hybridization between two species that were not closely related: an apomictic tetraploid male (2n = 4x = 36), and a sexual diploid female (2n = 2x = 18). The lack of affinity between genomes was clearly indicated by asynchrony during meiosis. The haploid genome (n = 9) showed unique behaviour, remaining univalent during prophase I and metaphase I, and undergoing sister-chromatid segregation and lagging at anaphase I. The laggard genome did not always reach the poles in time to be included in the telophase nucleus. However, when the inclusion was effective, this genome was distributed peripherally, changing the otherwise spherical nucleus shape. In the second division, the haploid genome behaved similarly, but as there was sister-chromatid segregation during the first division, the chromatids were slow to reach the poles, forming several micronuclei at telophase II. The two accessions were characterized as allo-autopentaploids, with the tetraploid genome (2n = 4x = 36) designated as B (from B. brizantha) and the haploid genome as X, representing a species with a distinct genome having little affinity with the B genome. Thus, the hybrids’ genome composition is represented by BBBBX. By comparing their meiotic behaviour with that observed in synthetic hybrids between B. brizantha and B. ruziziensis analysed previously, B. ruziziensis is the putative diploid sexual parent species in these pentaploid accessions.

Effect of hybridization of the Quercus crassifolia×Quercus crassipes complex on the community structure of endophagous insects

In a previous study, we showed that the geographic proximity of hybrid plants to the allopatric areas of parental species increases their morphological and genetic similarity with them. In the present work, we explored whether the endophagous fauna of hybrid plants show the same pattern. We studied the canopy species richness, diversity and composition of leaf-mining moths (Lepidoptera: Tischeridae, Citheraniidae) and gall-forming wasps (Hymenoptera: Cynipidae) associated with two species of red oaks (Quercus crassifolia and Quercus crassipes) and their interspecific hybrid (Quercusxdysophylla Benth pro sp.) in seven hybrid zones in central Mexico, during four seasons in 2 years. The study was conducted on 194 oak trees with known genetic status [identified by leaf morphology and molecular markers (random amplified polymorphic DNAs)], and the results indicate a bidirectional pattern of gene flow. Hybrid plants supported intermediate levels of infestation of gall-forming and leaf-mining insects compared to their putative parental species. The infestation level of leaf-mining insects varied significantly following the pattern: Q. crassifolia>hybrids>Q. crassipes, whereas the gall-forming insects showed an inverse pattern. A negative and significant relationship was found between these two types of insect guilds in each host taxa, when the infestation percentage was evaluated. It was found that 31.5% (n=11) of the endophagous insects were specific to Q. crassipes, 22.9% (n=8) to Q. crassifolia, and 8.6% (n=3) to hybrid individuals. The hybrid bridge hypothesis was supported in the case of 25.7% (n=9) of insects, which suggests that the presence of a hybrid intermediary plant may favor a host herbivore shift from one plant species to another. Greater genetic diversity in a hybrid zone is associated with greater diversity in the endophagous community. The geographic proximity of hybrid plants to the allopatric site of a parental species increases their similarity in terms of endophagous insects and the Eje Neovolcánico acts as a corridor favoring this pattern.