The variation of some leaf phenolics and isozymes in populations of British species of Limonium, and its relation to the breeding system

Abstract

The three species studied were L. vulgare Mill., L. humile Mill, and L. binervosum (G.E.Sm.) C.E. Salmon. They are self-incompatible, self-compatible and apomictic respectively. Using leaves sampled directly from natural populations of all three species, alcoholic extracts of leaf phenolics (mainly flavonoids) were studied by thin-layer chromatography. Population samples of L. binervosum were also chromatographed after cultivation. No attempt was made at chemical identification of the phenolics. The leaf enzymes esterase, leucine aminopeptidase and 6-phosphogluconate dehydrogenase of L. vulgare and L. humile only were studied by horizontal starch gel electrophoresis. Leaf enzymes of L. binervosum could not be detected, despite attempts using a variety of extraction techniques. Leaf morphology measurements were also taken from some population samples, particularly cultivated L. binervosum.
During a detailed consideration of the anticipated relationships between variation and the breeding system, a hypothesis particularly attributable to H.G. Baker was described. Stated simply, this was that variation would be found within populations of an outbreeding species, and that this variation would reduce distinctions between such populations. Conversely, for an inbreeding species there would be little variation within populations, but variation between these populations would be high. An apomictic species would show a similar pattern of variation to that of an inbreeding species.
An attempt was made to relate the observed biochemical variation within and between populations of the three species to this general hypothesis. It was found that the variation detected in some ways corresponded, and in other ways conflicted, with expectations. In the case of the chromatographic results, variation within populations conformed to the expected pattern, there being generally most variation in L.vulgare and least in L.binervosum populations, although some populations showed exceptions, and there was some overlap between species. However, chromatographic variation between populations was mainly the reverse of expectation, there being more variation between L. vulgare populations than L. humile. and the L. binervosum samples showing similar levels of interpopulation variation to L. vulgare.
These departures from expectations could be related to environmental influences on the production of leaf phenolics in wild material, and this was supported by the results from L, binervosum material cultivated in a uniform environment. Further, these cultivated samples showed a strong relationship between the breeding system and chromatographic variation within and between populations, and in addition this variation was related to the geographical origin of the populations and to leaf morphology.
The results from electrophoresis showed isozyme variation within L. vulgare populations, but little significant variation between them, but for L. humile the results were not as anticipated, there being a total lack of isozyme variation both within and between populations. The most likely reason for this was considered to be the action of the founder effect on populations established by self-fertilisation and long-distance dispersal.
If this was the case then there should also have been an absence of chromatographic variation between L. humile populations, but some variation existed, although probably caused by environmental factors.
It was also concluded that only under uniform growth conditions could chromatography of leaf phenolics be used as a reliable indicator of any genetic variation that may be present, but as has been shown by many other workers, electrophoresis of isozymes is a valuable tool for studying population and evolutionary genetics.