Discussion of case study results

The QAs occur in all the legume species included in this study but not in all individuals. This suggests additional evidence of the evolutionary role of secondary compounds, which are important for plant survival and reproductive fitness7 325. Thus, the mechanism for production of QAs has hereditary characteristics740'746'747. In some respects, the occurrence of alkaloids in legumes was disputed by previous literature. However, there was no dispute over whether QAs occur in Fabaceae. Bohlmann and Schumann742 observed that occurrences of QAs are restricted to the Sophorae, Podalyrieae and Genistae tribes. However, Aslanov et al.744 stated that a quinolizidine structure is found occasionally in molecules of complex alkaloids belonging to the indole, isoquilinone or other families of alkaloids not belonging to the quinolizidine series. Subsequently, Wink and Mohamed762 stated that the distribution of QAs is restricted to the genistoids, whereas many of the other legumes produce non-protein amino acids. This analysis of the QAs based on a N+-ring clearly demonstrates that plant individuals with QAs(+) were found in all the legume species studied. The frequencies of positive and negative alkaloid individuals certainly fluctuated according to plant species, confirming the validity of the study hypothesis. Therefore, the basic finding of this research is that QAs occur during a period of vegetation expansion and that their selectivity occurs not at the species level but rather according to individual, genetically regulated metabolisms. This is a logical explanation in the light of the fact that there are well-known alkaloid-rich and alkaloid-poor species of plants that have been developed by genetic selection (artificial evolution)740 746 747 748. There exists evidence that an alkaloid is plant specific and that the occurrence of QAs in plant individuals is connected with the metabolism of lysine. There exists a surplus of lysine in expanded vegetation, which leads to the production of QAs through the activity of HMT/HLTase and ECTase350. In individual plants without QAs, the biosyn-thetic pathway of the alkaloids with HTM/HLTase and ECTase is blocked351. Therefore, QAs and probably all other secondary compounds can be subjected to natural selection. These results clearly show that alkaloid distribution and frequencies among legume species and individuals indicate natural selection. Five groups of different frequencies [F(A), F(B), F(C), F(D), F(E)] have been found, and inside these groups different trends are apparent. This also supports the basic hypothesis of this study.

The distribution and frequency changes of alkaloids is a direct expression of natural selection. It is widely recognized that an understanding of biochemical diversity will facilitate the interpretation of the evolution of plants763 776. Moreover, evolutionary changes can occur rapidly and may be an important means by which species could escape extinction in the face of global change764. The fluctuation of QAs(+) and QAs(-) plants in legume populations is also relevant in this respect as QAs are known to be metabolism-protective secondary compounds7'736'765. The QAs(+} and QAs(-) plants are products of natural hybridization, which is based on ecological distribution and evolutionary diversification. Specifically, the occurrence of mutations as the result of the discrete changes in the mutant is evidence of diversification in short-term evolutionary dynamics. Although most of the species studied have only discrete MECs, there are clearly observed trends that show an increase in the number of QAs(+} plants in the case of 4 species and a decrease of this parameter in the case of 33 species. Three species had no tendency in this direction. As QAs are plant-protective compounds and also important for herbivores, a micro-evolution towards a decrease in the number of QAs(+} plants in a species can weaken the protective ability of the species and curtail the overall population size. In turn, this will certainly place pressure on herbivores and their populations. Moreover, some alkaloid-containing plant genera are often readily ingested by livestock767. In addition, plant toxins from some legumes have applications in human bio-medicine766. The micro-evolution of alkaloid distribution in legume plants is therefore a crucial topic.

Four species have positive MECs: A. arenarius, A. cicer, A. frigidus and L. polyphyllus. One might ask why these species differ in this respect to other legumes. Future studies will hopefully provide an exact answer to this question. Presently, it can be mentioned that these species do share some characteristics. For example, all of these species are perennials with very rapid growth-rates in Nordic ecosystems. They also have a very competitive capacity to proliferate in the areas where they grow. These plants also have a strong pollination-based mating system, being high rate outcrossers. On the other hand, the species with negative MECs (e.g. T. arvense and T. pratense) are common plants in Nordic ecosystems. They also have a pollination-based mating system and a high rate of outcrossing. Natural selection is probably in both cases connected with heterozygosity and other life characteristics of the species and with their modification reasoning. Darwin already recognized a difference between the types of modifications due to diversification768. Evolutionary trait variations in alkaloid distribution among wild and semi-wild Fabaceae species can also be discussed from this point of view. Of the 40 legume species and sub-species studied here, only 3 species did not have a tendency to change (micro-evolutionary coefficients of zero). According to Darwin's theory, if a species diversifies in order to occupy more places in the polity of nature, the change is depicted by relative distance along the horizontal dimension between different species. Alternatively, if a species is modified in order to become better adapted to its habitat within the polity of nature, there is no movement along the horizontal dimension768. In this sense, the species not perfectly adapted to their habitats and growing conditions are more disposed towards modifications and micro-evolutionary changes.

The variations in the distribution of the alkaloid ring in legume individuals is an evolutionary trait, which can be measured by an MEC. Prior to its development for the purpose of this study, this assertion has not been previously mentioned in the research literature. The question which follows is what do MECs, in fact, measure? They directly measure trends in trait change, and they indirectly measure the presence or absence of a mutational trajectory in the population of species. Therefore, MECs are useful coefficients for determining trends in the genetic oscillation of plants during a relatively short evolutionary period. Moreover, MECs also clearly manifest a tendency towards polymorphism in the biosynthesis of QAs(+), which has evolutionary implications. The genes that encode enzymes for the ring-closure step are repressed in alkaloid-negative plants and expressed in alkaloid-positive plants351. Polymorphism can create some difficulties in systematic plant diagnostics753. In addition, the current responses of ecological systems to climate may also represent an evolutionary role. It is possible that QAs(+} trends in individual plants can be regarded as a result of this kind of adaptive response in plant populations. Either way, QAs(+} and QAs(-) individuals in plant populations can be considered as evidence of current evolutionary responses by ecological and genetic systems.

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