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Reproductive Biology |
2CIRAD, Centre de Coopération Internationale de Recherche Agronomique pour le Développement, Avenue Agropolis, TA 74/0, 34 398 Montpellier Cedex 5, France; and 3Centre d'Ecologie Fonctionnelle et Evolutive, CEFE/CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France
Received for publication January 19, 2001. Accepted for publication May 29, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: apomixis • cytogenetics • Indian Ocean islands • Madagascar • microsatellite markers • reproductive biology • Rosaceae • Rubus alceifolius • Southeast Asia • weeds • SSR
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Frankham, 1997
). This reduction in genetic diversity is also caused by selection for certain traits in the mating system of the invading species, such as selfing or asexual reproduction. In fact, selfing species, or species with intense asexual reproduction, predominate among effective colonizers, as has been noted for plants (Allard, 1965; Baker, 1967; Brown and Marshall, 1981; Price and Jain, 1981; Husband and Barrett, 1991), as well as for animals, such as parthenogenetic geckos (Cuellar and Kluge, 1972), slugs (Selander, 1983), and snails (Doums, Perdieu, and Jarne, 1997
; Viard, Justy, and Jarne, 1997
; Ostrowski, Jarne, and David, 2000
). Such traits are preadaptations that appear to facilitate colonization by species that already possess them, and in other species, island colonization has been suggested to be accompanied by shifts to higher levels of inbreeding or clonal growth (Barrett and Richardson, 1986
; Barrett and Husband, 1990
). However, few studies have documented major changes in mating systems of invading species in areas of introduction.
Such a shift in mating system was suggested by previous information available on a serious weed in several Indian Ocean Islands, the bramble Rubus alceifolius. A comparative study using AFLP (amplified fragment length polymorphism) markers of the genetic diversity of R. alceifolius (Amsellem et al., 2000)
, through out the plant's native range (from northern Vietnam to Java) and in its areas of introduction (in the Indian Ocean islands of Madagascar, Mayotte, La Réunion, and Mauritius, and in Queensland, Australia), showed a considerable reduction in genetic diversity in all areas of introduction, with the exception of Madagascar. In Madagascar, Vaughan (1937)
considered the Rubus on the island to include an endemic species, which he named Rubus roridus, that differed from R. alceifolius in stipules and hair morphology. Later, other botanists ranked R. roridus as a simple ecotype of R. alceifolius (Owadally, 1960
; Rivals, 1960
; Kalkmann, 1993
).
Within the Indian Ocean Islands, each population sampled was characterized by a single clone, and each clone was closely related to a subset of individuals from Madagascar. Successive nested founder events thus appear to have resulted in cumulative reduction in genetic diversity followed by the spread by asexual means of a single principal genotype in each invaded island. However, plants on these islands produce large numbers of fruits and seeds (plants do not set any seeds if pollen is excluded, L. Amsellem, personal observation), and areas occupied by the plant are characterized by a dense soil seed bank (Thébaud, 1989
; Lobet and Triolo, 1999
). Variation in reproductive characters and AFLP markers suggest that in the invaded islands, a shift may have occurred from sexually to apomictically produced seeds. This hypothesis is supported by the fact that apomixis has been already documented in Rubus (Thomas, 1940
; Gustafsson, 1943
; Einset, 1951
; Pratt and Einset, 1955
; Aalders and Hall, 1966
; Nybom, 1988, 1995
).
In the present study, we investigated whether such shifts in the reproductive system have occurred during the invasion of Indian Ocean islands by R. alceifolius. Microsatellite markers were developed (Amsellem, Dutech, and Billotte, 2001)
to investigate the reproductive biology of this weed, because they are multiallelic, codominant, and particularly adapted to discriminate among closely related genotypes (Plaschke, Ganal, and Röder, 1995
). We conducted experiments to test for the occurrence of apomixis in R. alceifolius both in its native range and in areas of introduction, through comparison of genotypes of half-sib progenies with those of their maternal parents. Moreover, as apomictic species are known to have a large proportion of empty pollen grains due to disruptions during pollen meiosis (Nybom, 1985, 1988, 1995
; Richards, 1986
), cytological observations of pollen grains were also performed within populations in the area of introduction to confirm our observations.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The precise origin and time of introduction of R. alceifolius into La Réunion and Mauritius are still unknown, but date of introduction is thought to be about 1850.
Populations studied
In the native range
We studied one population, located near Lang Son (22°05' N, 106°38' E), in northeastern Vietnam. In this population, several fruits from open pollination were collected from different ramets on the same individual, from which leaves were also collected to determine the maternal genotype. Nine half-sib seedlings obtained from these seeds were grown in a greenhouse. Seeds from other individuals were collected, but unfortunately had to be discarded because no leaves of these other individuals were sampled by the plant collector. Although this limited sample allows no estimation of population level variation of sexual reproduction in relation to other modes, if differences in genotypes between maternal parent and half-sib progeny are in fact observed, this finding would at least demonstrate that sexual seed production occurs for this individual.
In the area of introduction
In six different localities from the eastern coastal lowlands of Madagascar, we sampled 1–5 individuals and seeds produced by them in 1–5 populations per locality, each population being 
5 km from other populations in the same locality. A total of 87 individuals were sampled. These were screened to identify individuals with rare alleles that would facilitate comparison of maternal genotypes with those of half-sib progeny.
Three Madagascan individuals and their half-sib progenies were selected. Two of these individuals were chosen in the locality of Bengaly, from populations 15 km distant from each other (individuals B13 and B51). The third individual was chosen in the locality of Ivoloina (individual I12) . Sample sizes for the half-sib progenies of B13, B51, and I12 were n = 30, 29, and 22 seeds, respectively. For each maternal parent, half-sib progeny consisted of a mixture of seeds taken from several fruits harvested from different branches.
In La Réunion, we performed experimental pollination in a population on this island in order to test directly for apomixis, using parents with unique patterns of microsatellite markers not found in the rest of the population. These experiments were conducted in the district of Grand-Etang (480 m above sea level, 21°04'58'' S, 55°40'30'' E), near Saint-Benoît. This population is large; flowers are plentiful and the flowering season is prolonged. In a first survey, 20 individuals were sampled along a transect following the road; each individual was separated by a minimum distance of 3–4 m from the previous one. Genotypes of the parents were determined at eight microsatellite loci (Amsellem, Dutech, and Billotte, 2001)
. In addition to the one dominant monomorphic microsatellite pattern observed in this patch, nine individuals clearly showed variation either at a single locus at which genetically variant individuals were homozygous or at two loci (individuals bearing a homozygous mutation at one locus and a heterozygous mutation at another one). In order to trace their diagnostic alleles in half-sib progenies, double genetically variant individuals were used only as pollen receivers, whereas nonvariant and simple genetically variant individuals were used both as pollen donors and receivers.
Flowers of individuals chosen as parent individuals were then bagged at the bud stage to prevent any uncontrolled pollination or contamination of anthers with unwanted pollen before experiments.
Each inflorescence of R. alceifolius includes 12 hermaphroditic flowers. Each flower possesses 100 separate styles and stigmas, which are much longer than the stamens disposed around them. During 10 d, bags enclosing flower buds were inspected every morning, and flowers were pollinated just after opening, using freshly opened flowers as pollen sources. Preliminary studies (T. Pailler, Université de la Réunion, unpublished data) show that flowers are receptive as soon as they open. Male flowers were obtained by cutting styles with scissors at the height of stamens. These flowers were then taken and carefully rubbed onto the styles of the flowers chosen to receive pollen. One sepal of each pollinated flower was then painted on its external face with acrylic paint, to mark the manipulated flowers within the inflorescence (sepals are persistent). Pollinated flowers were then bagged again to prevent any uncontrolled pollination after manipulation.
A total of 44 flowers were manually pollinated. After each individual flowering period, flowers that were still on the maternal parent were then bagged in mosquito net sleeves. Fruits were collected once ripe.
Germinated seeds were grown in a greenhouse and seedlings were harvested 1.5 mo later. Their microsatellite profiles were obtained as described in Amsellem, Dutech, and Billotte (2001)
and were compared to those of the maternal parent and the pollen donor.
Microsatellite markers
Rubus alceifolius is tetraploid (Amsellem, Chevallier, and Hossaert-McKey, in press
). Because it is not known whether the plant is allo- or autotetraploid, we were not able to analyze the microsatellite profiles we obtained using classical methods of population genetics. Indeed, in the absence of information on the type of tetraploidy, it is impossible to link alleles to one or the other homeologous loci observed. The analysis was thus restricted to a comparison of genotypes of half-sib progeny to that of their maternal parent, to determine whether sexuality occurs, by the presence or not of alleles not found in the maternal parent.
For all localities studied, maternal genotypes and genotypes of their half- sib progeny were determined for the same eight polymorphic microsatellite markers studied by Amsellem, Dutech, and Billotte (2001)
.
AFLP markers
In Madagascar we also conducted an AFLP analysis on the half-sib progeny of individual B13, in order to have an overview of its genetic diversity and for comparison with previous results (Amsellem et al., 2000)
. We used commercial AFLP kits (GIBCO-BRL) and followed methods used by Amsellem et al. (2000)
. The four primer pairs used were E-AAC/M-CAA, E-AAC/M-CAT, E-AAC/M-CTA, and E-AAC/M-CTT.
Pollen aspect
Pollen viability can be a good indicator of disruptions during meiosis in pollen development of apomictic species, as has been shown in other species of Rubus (Nybom, 1985, 1988, 1995
; Richards, 1986
). If such disruptions occur, a fraction of pollen grains will be empty of cytoplasm and nonfunctional, whereas functional pollen will be full of cytoplasm. To examine condition of pollen grains from Reunionese individuals, they were harvested from six flowers, each flower being haphazardly chosen from one individual during manual pollinations.
Pollen grains were stained with cotton blue (lactophenol) and observed with an optical microscope. We counted 143–179 pollen grains per flower, determining the fractions of pollen grains empty and full of cytoplasm for each of the six flowers. A total of 971 pollen grains was counted. To determine whether the proportions of the two types were significantly different from each other we applied a 
2 test on the whole sample of pollen grains.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, except for two individuals (which were also among the 12 variant individuals detected with microsatellite markers): descendant B13–15 (polymorphic with primer pair E-AAC/M-CTA) and descendant B13–23 (polymorphic with primer pair E-AAC/M-CAA). Those differences consisted of one band level out of the total of 491 levels for each descendant. No segregation was found among the eight microsatellite loci between the individual B51 and its half-sib progeny, although this mother plant possessed unique variation in the homozygous state on locus mRaCIRRIV1E8, compared to the rest of the population.
Of the half-sib progeny of individual I12, four individuals were found to have genotypes different from that of their maternal parent, only at locus mRaCIRRV2F4. No segregation was found either at locus mRaCIRRI1G3, for which the maternal parent was found to be heterozygous, or at locus mRaCIRRI1D3. To illustrate that the heterozygosity considered here is "allelic" and not a fixed heterozygosity (band levels would then represent homeologous loci and not alleles of a single physical locus, as could plausibly be expected in an allotetraploid), Fig. 1 shows the autoradiography of the microsatellite locus mRaCIRRI1G3 for individual I12 and its half-sib progeny. Variation in the "low" locus between the maternal parent and the half-sib progeny clearly represents alleles of a single physical locus and not homeologous loci. In this figure, it is also clear that no Mendelian segregation occurred among the offspring studied.
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2 test showed that in the flowers we studied, nonfunctional grains and functional ones occur in equal proportions (2 = 0.03; df = 1; P = 0.86). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, sexual reproduction seems to be the exclusive method of reproduction in the native range of R. alceifolius. Moreover, in La Réunion, our observations of pollen grain viability showed that half of the pollen grains within stamens are empty of cytoplasm. These results strongly suggest that R. alceifolius has switched from sexual reproduction to apomixis during the colonization of the Indian Ocean islands.
In fact, among the 12 subgenera commonly recognized in the genus Rubus, apomixis is frequent in the European subgenus Rubus, but only in polyploid taxa (Gustafsson, 1943
; Nybom, 1986
; Weber, 1995
). The reproductive biology of tropical species of the genus Rubus has been only briefly investigated. According to Nybom (1986)
and Thompson (1997)
, Asian species of the subgenus Malachobatus, to which R. alceifolius belongs, are mostly polyploid and produce seeds exclusively sexually. However, R. alceifolius was not represented in these studies. Proportions of fully functional pollen grains of the species already investigated (60–95%) are higher than those we found in the present study. The highest values for this biological trait were described for R. moluccanus (95%), which is considered to be the sister species of R. alceifolius. Busemeyer et al. (1997)
showed that natural populations of R. moluccanus in the Philippines are genetically very diverse and that gene flow within and among populations is high. Similar conclusions are suggested for R. alceifolius in its native range by a previous study of genetic diversity of the plant (Amsellem et al., 2000)
. The results on the half-sib progeny of the Vietnamese individual obtained in the present study (all seeds were strictly produced through allogamy), combined with a brief study of genetic diversity in Vietnamese and Thai populations using microsatellite markers (Amsellem, Dutech, and Billotte, 2001)
point in the same direction. However, these results must be confirmed with experiments employing controlled crosses and conducted in a number of different populations with large numbers of descendants.
Evidence for a switch in the reproductive biology of R. alceifolius in the area of introduction
Madagascan half-sib progenies showed very little variation from the genotypes of their respective maternal parents (12 variant individuals of the 81 considered in the 3 half-sib progenies, or 14.8%), even though plants chosen as maternal parents bore microsatellite alleles that were relatively rare in their populations. Variations observed between maternal parents and their progeny could result either from sexual reproduction or somatic mutations.
Variations detected in Madagascan half-sib progenies are consistent with the stepwise mutation model (Schlötterer and Tautz, 1993
). However, the large proportion of variant individuals found in the half-sibs (23.3% and 18.2% of variants presenting this type of variation in half-sib progenies of B13 and I12, respectively) is not compatible with the mutation rates of microsatellite markers described in the literature, ranging from 10–6 to 5.10–4 (Tachida and Izuka, 1992
; Valdes, Slatkin, and Freimer, 1993
). Such a hypothesis about the origin of variations observed in Madagascan half-sibs, if plausible, seems thus not to be exclusive.
Variation seems thus to result largely from sexual reproduction. The results are not compatible with production of seeds by autogamy, because in this case there would have been M endelian segregation at heterozygous loci in maternal parents. This pattern was not found at such loci for half-sibs produced by B13 and I12 (as seen in Fig. 1 for individual I12). The most plausible explanation is that the observed variation resulted from allogamous crosses involving pollen bearing variant alleles. This variant pollen could come, for example, from an individual (in the same population or a neighboring population) not sampled during the screening. However, the respective contributions of mutation and recombination in producing the observed variation are unknown.
Our observations of pollen grains empty of cytoplasm suggest a high degree of disruption in meiosis during pollen development. Disruptions accompanying apomixis are known to include formation of multivalent, univalent, and sometimes inversion bridges (Thomas, 1940
; Gustafsson, 1943
; Craig, 1960
; Bammi and Olmo, 1966
; Nybom, 1988
). There may be a link between cytology and the degree of sexuality: The less similar the homeologous genomes of an allopolyploid species, the more regular meiosis is likely to be, and thus the higher the degree of sexuality realized (Thomas, 1940
; Nybom, 1988
). However, apomixis may also occur even when meiosis is not disrupted (Gustafsson, 1943
).
The low proportion of Madagascan half-sibs showing variation compared to their maternal parents, the clonal pattern obtained for a Madagascan half-sib progeny with AFLP markers, the apparent absence of fertilization in Reunionese progeny, and the large proportion of empty pollen grains in Reunionese plants (indicating irregular meiosis) all strongly suggest that in the islands where R. alceifolius has been introduced, a very large proportion of seeds are produced asexually and that sexual reproduction is infrequent (Madagascar) or perhaps even absent (La Réunion). The lack of analogous data on numbers of functional and nonfunctional pollen grains in flowers from the Asian native range means we cannot definitively conclude that a switch in that biological trait occurred during or following introduction of R. alceifolius into the Indian Ocean islands.
Rubus alceifolius was found to be monoclonal within all Indian Ocean islands other than Madagascar, the source of these secondary introductions (Amsellem et al., 2000)
. The hypothesis of a sexual reproductive biology associated with the short period of time that had elapsed since the introduction of R. alceifolius in these islands (150 yr ago for La Réunion and Mauritius [in Lavergne, 1978] and 50 to 100 yr ago in Queensland [E. Bruzzeze and A. Kirk, CSIRO, personal communication]), and the possibility of several independent introductions, would not allow fixation of a genotype.
Possible scenarios to explain a switch towards apomixis in the area of introduction
Owing to our low sample size of half-sib progeny in the species' native range, we cannot firmly conclude that a switch in this biological trait occurred in the area of introduction. Thus, it is possible that the plants introduced into the Indian Ocean Islands may have originated from unsampled apomictic lines from the species' native range. However, if a proportion of the offspring of a sexual plant carries some genes for apomixis, the offspring of an obligate apomict plant only carries apomixis genes. Thus, population genetic models show that if all else is equal, an apomict plant always replaces its outcrossing sexual variety after a few generations (Marshall and Brown, 1981
), and prior studies of genetic diversity, using both AFLP and microsatellite markers (Amsellem et al., 2000
; Amsellem, Dutech, and Billotte, 2001)
, didn't indicate apomixis in the plant's native range. Other factors, such as difficulty in pollination or in finding mates, might have favored the capacity for asexual seed production. Also, in an ecosystem without the adapted enemies occurring in its native range, Red Queen pressures favoring sexuality are likely to have been relaxed (Jaenick, 1978
; Lively, 1987
; Ebert and Hamilton, 1996
). The higher level of genetic diversity found in Madagascar suggests that genetically different individuals may have been introduced on that island. Pollination is entomophilous (pollinators such as honey bees, Apis mellifera, and a wasp, Polistes hebraeus, are present on these Indian Ocean islands and visit numerous individuals of R. alceifolius [Thébaud, 1989
, for La Réunion]), and R. alceifolius is potentially autogamous (S. Maurice and D. Strasberg, University of La Réunion, personal communication; L. Amsellem, unpublished data). Thus, difficulty in finding mates does not seem a plausible explanation for asexual seed production in Madagascar. These characteristics, as well as the short period since the introduction of R. alceifolius in Madagascar (from a few millennia to a few centuries ago), are not compatible with the "sudden" appearance of apomixis in Madagascar. The quasi-loss of sexuality of R. alceifolius in the islands where it has been introduced could be a consequence of interspecific hybridization. Its fixation might be also due to some selective advantage or simply represent a consequence of hybridization with no particular adaptive significance in these insular populations.
It is interesting to note that some simple-leafed Madagascan Rubus were considered by Vaughan (1937)
to constitute an endemic species, Rubus roridus (Lindl.). Alternatively, these may simply represent an ecotype (Owadally, 1960
; Rivals, 1960
; Kalkmann, 1993
; Amsellem et al., 2000
). This uncertainty is interesting. The subgenus Malachobatus, to which R. alceifolius belongs, is known only from Southeast Asia (Van Thuan, 1968, 1970
; Kalkmann, 1993
; Friedmann, 1997
). Genetically differentiated populations belonging to a species from this subgenus in Madagascar could come from a remote introduction. Rubus roridus could represent populations differentiated from an ancient introduction of R. alceifolius (for example, carried by proto-Madagascan humans arriving 3000 yr ago [Wigboldus, 1994
]). Evolution and divergence of R. roridus on this island could have been insufficient to completely bar gene flow, but sufficient to create cytoplasmic incompatibility with recently introduced R. alceifolius in the event of hybridization between the two. This could have given rise to a genetically diversified stable hybrid taxon. Such stable hybrid taxa are known in the genus Rubus (Gleason and Cronquist, 1991
; Weber, 1996
). In this context, it is significant to note that many agamospermous plant species are derived through of hybridization (Manton, 1950
; Stebbins, 1950
; Kirchner and Stepanek, 1996
; Holsinger, 2000
).
An alternative hypothesis is that individuals from several geographically, cytologically, and genetically divergent populations of R. alceifolius in the species' native range in Asia were introduced in Madagascar. Hybridization between these divergent populations brought into contact in Madagascar could have led to a hybrid taxon with disruption of meiosis and appearance of apomixis.
The paradoxical association in Madagascar of high genetic diversity (Amsellem et al., 2000)
and occurrence of apomixis could support the hybridization hypothesis. In such a case, a hybrid swarm covering all the genetic intermediates between the two putative parents might be expected. This could explain the presence of several genetically distinct clonally reproducing lineages. Moreover, the low level of sexual reproduction suggested by our analysis of Madagascan half-sibs may be sufficient to maintain variation in a predominantly apomictic population (Watkinson and Powell, 1993
; Kraft and Nybom, 1995
; Kraft, Nybom, and Werlemark, 1996
; Kollmann, Steinger, and Roy, 2000
).
Another possible scenario to explain monoclonality of R. alceifolius in other Indian Ocean islands is that several genotypes could have been first deliberately introduced into each island (for therapeutic properties, as ornamentals, or as botanical curiosities) several times independently. Then, among all the genotypes present, a particularly aggressive genotype for these insular environments (one in La Réunion and a very similar one in Mayotte, Mauritius, and Queensland [Amsellem et al., 2000
]) could have been selected, apomixis permitting the conservation of favorable gene combinations.
Conclusion
The evidence presented here of a switch in the reproductive biology of R. alceifolius between its native range and islands where it has been introduced—a switch from sexual seed production to apomixis—is the first evidence for asexual seed production for any Rubus species from the subgenus Malachobatus. The estimation that only 15% (at most) of the seeds produced in uncontrolled crosses in Madagascan populations were sexually produced is consistent with another study of the reproductive biology of two pseudogamous aposporous European species of Rubus, in which estimated proportions of sexually produced seeds were 14 and 17% (Kollmann, Steinger, and Roy, 2000)
. To characterize the type of apomixis occurring in the introduced range of R. alceifolius, other experiments must be done, accompanied by both embryological and cytological observations.
Apomixis may have allowed a particularly well-adapted and aggressive genotype of R. alceifolius to spread through these islands and to invade their native plant communities, which are known to be susceptible to invasions (Pickett, 1976
; Mueller-Dombois, 1981
). Our study suggests that the change in the reproductive biology of this weed took place in Madagascar, although more detailed studies on this island are necessary to validate this hypothesis. The hypothesis of the origin of an asexual hybrid in Madagascar must be confirmed, either by a fine taxonomic screening of individuals from wild populations or through controlled pollination experiments.
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FOOTNOTES |
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4 Author for reprint request (hossaert{at}cefe.cnrs-mop.fr
).
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