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As we approach the 150th anniversary of the publication of Charles Darwin's seminal work, it is appropriate that we again discuss speciation mechanisms. Speciation often occurs when populations become separated from each other. Separation may be geographically obvious but it may also take more subtle forms. One such example is seen in the preferences for different host plants in different populations of parasitic species, as has been studied by Thorogood et al. (Bristol University and Natural History Museum, London, pp. 1005–1014). Orobanche minor is a parasite with a very broad host range, but within that range specific varieties or sub-species of the parasite may exhibit specific host preferences. The authors have examined the parasitic behaviour of O. minor var. minor and O. minor ssp. maritima that usually infect, respectively, red clover and sea carrot. In reciprocal infection experiments, seeds of O. m. maritima germinated in the presence of both host species; the germination percentage was much higher with sea carrot than with red clover and only with sea carrot was there any attachment of the seedlings to the host root. Orobanche m. maritima therefore did not infect red clover, showing an early selection of its normal host. Surprisingly, O. m. minor showed much better germination and attachment with sea carrot than with its usual host, red clover. However, its ‘performance’ as a parasite, as measured by parasite biomass, by the formation of flowers and by effects on host biomass, was much better with red clover than with sea carrot. Thus in O. m. minor, host selection occurs at a later stage than is seen with O. m. maritima. These data, combined with the authors' earlier demonstration that the two varieties exhibit genetic differences, indicate speciation in progress. Further, because O. minor is a self-fertile species, genetic differences between varieties will be reinforced by the strong tendency to inbreeding, thus accelerating the speciation process.
Submerged aquatic plants have a problem in obtaining CO2 for photosynthesis. Although CO2 concentration in water is much higher than in air, the low gas diffusion rate in water more than cancels this out. Some species adapt to submergence by induction of CAM, effectively a CO2-concentrating mechanism (e.g. Littorella uniflora: Groenhof AC, Smirnoff N, Bryant JA. 1988. Journal of Experimental Botany 39: 353–361). Another adaptation shown by plants of isoetid life form is the possession of aerenchyma, which brings CO2 from the sediment via the roots to the photosynthetic organs. The question asked by Winkel and Borum at the University of Copenhagen (pp. 1015–1023) is whether non-isoetid submerged species can also use sediment CO2. They selected four non-isoetid aquatic species (Lilaeopsis macloviana, Ludwigia repens, Vallisneria americana, Hydrocotyle verticillata) and, for comparison, one isoetid (Lobelia dortmanna). Plants were placed in two-compartment split chambers; the CO2 concentration around the roots was manipulated independently of that around the shoots. Thus rates of photosynthesis using CO2 supplied to the roots at various concentrations could be compared with the rates obtained when leaves were supplied with CO2 at different concentrations. Of the five species, only Hydrocotyle was totally unable to use root-supplied CO2. Of the others, Lobelia was, as expected, the most efficient, but two of the non-isoetids, Lilaeopsis and Vallisneria, were also able to obtain up to 75% of their CO2 requirement from the sediment when the CO2 concentration in the water was in equilibrium with air. All four users of CO2 from sediment have low shoot : root area ratios, in contrast to Hydrocotyle in which the ratio is much higher. However, utilization of CO2 via the roots in Ludwigia is much lower than would be expected from its shoot : root ratio. Use of 14C suggested that in Ludwigia, utilization of CO2 from sediment is limited by rates of uptake and/or transport. In conclusion, sediment CO2 is likely to play a much larger role in the carbon economy of submerged plants than had been suspected.
‘Biological oddities have traditionally attracted the attention of naturalists’, write de Vega et al., at Sevilla, Spain (pp. 1065–1075). I am sure that we all agree, but I would extend ‘naturalist’ to include anyone with a strong interest in biology. It is indeed fascinating to see what evolution can lead to in terms of life form, life style, habitat use and so on. Thus we consider those most bizarre parasitic plants that live as endophytes, totally within their hosts except when flowering. Flowers are produced from within the host's root systems and appear at ground level, the only sign visible to the ‘outside world’ of the parasite within its host. Rafflesia is the obvious example, but in fact there are four different plant families of endophytic parasites. The authors worked with a Mediterranean species, Cytinus hypocistis, and wanted to identify its pollination agents. This species is self-fertile but is monoecious, so self-pollination requires pollen transfer from flower to flower, albeit on the same plant. Long-term, detailed and patient field observations were made, showing that ants were responsible for 97·4% of floral visits. Surprisingly, very few flying insects visited the flowers and only one of these, the fly Oplisia aterrima, was a pollinator. Five species of ant were regular visitors and another five were less regular. Ants were observed carrying pollen between flowers; pollen viability was slightly reduced after contact with ants; nevertheless fruit set was between 80 and 90% in plants from which other pollinators had been excluded. This may be compared with the 96–99% seen in open-pollinated plants, confirming that despite the infrequency of its visits, Oplisia aterrima did participate in pollen transfer (wind-pollination did not occur). However, it is abundantly clear that ants are the major pollinators, being rewarded with nectar during the process. This contrasts with South African Cytinus species, which are pollinated by small mammals.
The reversion from self-incompatibility to self-fertility has occurred many times in the evolution of angiosperms. This reversion is totally dependent on the breakdown of the self-incompatibility system, based on recognition of the products of self and non-self S-alleles. Further, there are several species in which there is partial self-incompatibility. An example studied by a Canadian–Mexican group, Ferrer et al. (pp. 1077–1089) is Flourensia cernua (Asteraceae), a shrub of the Chihuahuan desert, Mexico. Space permits discussion of only a limited part of the authors' comprehensive and fascinating paper. In two populations of F. cernua, one from high and one from low scrub density, different levels of self-incompatibility were detected by comparing seed set in hand self-pollinated and hand cross-pollinated plants. Strongly self-incompatible (SI) plants made up approx. 50% of the population; at the other end of the scale, 20% were self-compatible (SC). However, the situation was more complex than this. Strongly SI plants set very few seeds, either in hand cross-pollination or in open pollination. This may indicate pollen limitation, possibly based on a limited range of S-alleles in the population. In the other classes, seed set was higher after hand cross-pollination than after open pollination. Interestingly and perhaps unexpectedly, seed set in open-pollinated plants was greater in the low density scrub than in the high density: pollen ‘quality’ was higher. Further, in plants in which there was a total or partial breakdown of self-incompatibility, out-crossed seed set, despite the high level of self-compatibility, was higher than in strongly SI plants. There were also effects on seed fitness: germination rates were lower in seeds arising from open pollination in the SI plants than in the partially or totally SC plants. The authors suggest that partial self-incompatibility is selected for in F. cernua but that nevertheless, the level of inbreeding is relatively low. Partial breakdown of self-incompatibility may thus be a strategy to increase reproductive output especially when colonizing new habitats.