A historical theory for why imperfect flowers might be beneficial was that it could promote out crossing. Clearly a dioecious species is an obligate out crosser, since the male and female flowers exist on separate plants. However, it is not so clear that this would be true for monoecious plants. It was hypothesised that if monoecy were to promote out crossing inherently, monoecious plants would not have to rely on other costly mechanisms to promote out crossing such as molecular self-incompatibility . Studies have not shown this to be the case, indeed monoecy is just as common in self-compatible species as it is it in SI species5 , though of course this does not rule out the possibility that avoiding self-fertilisation could still be a factor in the evolution of imperfect flowers, especially for dioecious plants. Instead of making a theory and trying to find data to support it, it is more fruitful to look at the natural world and make a theory to explain it. For this, we might ask: “what kinds of plants are likely to bear imperfect flowers?”. Analysis of large datasets has shown that species with imperfect flowers are likely to be viniferous or woody, wind-pollinated plants with small green flowers, and for dioecious plants, the bearing of fleshy fruits2 . One theory proposed by Bertin to explain enrichment in wind-pollinated plants is the relatively low cost of their flowers allows for monoecy to develop. Monoecy is more expensive than perfection because a plant with unisexual flowers will have to produce two sets of flowers where asingle perfect flower would suffice, thus Bertin argues, it is only when flowers are relatively cheap that monoecy can be permitted. Bertin’s theory does not explain why monoecy might be desirable, instead it merely states why it might not be punished. Since monoecy is not uncommon in plants, there must be some kind of advantage. Again, we should turn to direct observations of the natural world to explain this. As we have seen,vertical grow bench wind pollinated plants are more likely to evolve monoecy than insect pollinated. Indeed, all gymnosperms are wind pollinated and can be thought of as dioecious or monoecious since they bear micro- and macrostrobili.
When we consider the flowers of a wind pollinated angiosperm tree such as hazel, the benefit of the separation of the sexes becomes immediately apparent. The benefit is specialisation. Male hazel inflorescences hang from the branch; their primary role is in dispersal of gametes and their hanging habit allows for the efficient dispersal of pollen by the wind. On the other hand, the primary role of female flowers is the reception of gametes – female hazel flowers fulfil this end by having tentacle like protrusions that catch pollen from the air. Insect pollinated plants have no such need for specialisation – in fact they need the same insects to visit both the male and female flowers. While Bertin’s theory may explain how monoecy is permitted, the advantages bestowed by specialisation explains how it can become fixed. It has further been proposed that pistils that are separated from stamens might be more effective in being pollinated because they avoid “pollen interference”. Self-pollen could clog the tip of the pistil, stopping pollen from other plants from reaching into the stigma. It could also begin to grow down into the stigma and even if it is stopped at the micropyle the pollen tubes could partially or completely block the micropyle, again interfering with fertility mechanistically. This would be especially true for wind-pollinated plants that produce copious amounts of pollen that fills the surrounding environment, fitting with the observation that wind pollinated plants are more likely to have imperfect flowers. Despite the elegance of this theory, it remains to be confirmed, and studies in gynodioecious Plantago maritimax and representatives from the Lamiaceae8 failed to find evidence for an female advantage due to lack of interference from self-pollen. Finally, the potentially high cost of female flowers has been suggested as a driver towards imperfection. Hermaphroditic flowers must produce nectar, petals, sepals etc., stamens, ovules and fruits and seeds. A male flower must produce a + b, while a female flower must produce a + c + d. Even if the fruits and seeds are aborted at some stage, there is still the cost of their inception, for example the monoecious Cucumis spp. make fairly large prefruits on their female flowers long before pollination. When fruit and seeds are especially large , unisexual flowers can allow a plant to produce pollen without committing large reserves of resources to ovules, fruits and seeds.
As we shall see, andromonoecy is more common than gynomonoecy, which fits with this theory.If our starting point is a perfect flower as seen in most angiosperms and the basal Amborella trichopoda and Magnoliidae, and the unisexual flower is the derived state what are the intermediary steps from hermaphroditic flowers to a monoecious or dioecious species? Protogyny and protandry, androand gynomonoecy, along with andro- and gynodioecy, present themselves as intermediaries between perfection and monoecy or dioecy. Almost all of these states have been shown experimentally to be intermutatable. Once these states arise, they can remain environmentally plastic, or become fixed: in monoecy by the evolution of strict developmental pathways; and in dioecy by the evolution of autosomal sex determining loci or true sex chromosomes.One of the first steps towards unisexual flowers is thought to be the temporal separation of male and female function in hermaphroditic flowers6,9 . This separation is most often thought of as a strategy for promoting outcrossing and/or reducing pollen interference. The temporal separation of the sexes allows them to begin evolving as separated pathways, and can eventually lead to true unisexual flowers.C. bulbosum is an annual from the Apiaceae common by riversides in Europe. The plants are andromonoecious – bearing both hermaphroditic and male flowers in their umbelliferous inflorescences. Andromonoecy is common in the Apiaceae, and appears to have evolved multiple times from dioecy within this group. Some Apiaceae are protandrous , highlighting the role this sexual form may play in the evolution of andromonoecy. C. bulbosum has been shown to respond to damage and pollination restriction by increasing its ratio of hermaphroditic to male flowers in order to ensure seed set. Thus, this plant is able to respond to environmental cues by changing its sex ratios to enhance reproductive success. Andromonoecy is thought to be a more common first step towards monoecy than gynomonoecy because the benefits of avoiding commitment to female flowers is more obvious, as discussed above.In androdioecious populations, both hermaphroditic and male plants exist.
It has been proposed that in such a situation, the population will naturally tend toward true dioecy because hermaphroditic flowers cannot compete with the high pollen production of exclusively male flowers and will therefore tend to become more female. While this has not been shown experimentally, the inverse has. By removing all male plants from a population of M. annua, pollen production by the hermaphroditic plants was shown to increase over 3 generations as compared to the control group12, implying that the hermaphroditic flowers are compensating for a lack of pollen. This implies that the hermaphrodites in the wild, unmanipulated populations are restricting their pollen production and perhaps also enhancing female function as a response to the males in their population. This may mean that M. annua is providing us with a snapshot in evolutionary time of a species on its way towards evolving dioecy .Gynomonoecy, where plants bear female and hermaphroditic flowers, is a very rare sexual state. In a survey of Occupied Palestine, Sinai and Jordan, only 0.4% of plants were listed as gynomonoecious. Further,vertical grow cannabis most gynomonoecious plants are of the Asteraceae, where female ray florets surround perfect disc florets. It is not likely that the femininity of these ray florets underlies their evolution and maintenance – rather it seems that these are specialised flowers that serve as pollinator attractors. Overall, gynomonoecy has been understudied as a sexual system in plants, partially due to its rarity and partially because it is not thought to be important as a sexual system. Gynomonoecy does appear in domesticated species such as cultivars of Spinacia olearacea, which is usually monoecious, showing that gynomonoecy represents a possible route between sexual systems in plants .Given the rarity of gynomonoecious species, it is perhaps surprising that gynodioecious species are so prevalent. Perhaps specialising as female on a separate plant is more beneficial than doing so on the same plant. Further, Given the rarity of gynomonoecious species, it is perhaps surprising that gynodioecious species are so prevalent. Perhaps specialising as female on a separate plant is more beneficial than doing so on the same plant. Further, Monoecy in Zea mays is rigidly controlled with an apical male inflorescence and axillary female inflorescences. This is not simply a relic of domestication – teosinte also bears its female inflorescences laterally to its male inflorescences. Likely, this spatial separation and elevation of the male tassel enhances the specialisation of these inflorescences. Monoecy does not have to follow this strict habit though. In Cucumis spp. the flowers alternate unpredictably between male and female along the shoot. Another vine, Freycinetia funiculari, preferentially makes male flowers early in its life before switching to female flowers later in development.
Postponing growth of female flowers makes sense because of the greater resource investment required, especially for vines which may spend years beneath the canopy waiting for full sun. Charles worth and Charles worth briefly discuss the evolution of monoecy. They argue convincingly that monoecy must evolve from hermaphroditism through a series of at least two mutations, through the intermediary steps of gyno- or andromonoecy. Charles worth and Charles worth further postulate that these genes would have no tendency to be linked, unlike the mutations leading to dioecy which must be linked. Confirmation of this theory comes from Erin Irish’s work in stacking the maize mutations of the tasselseed and dwarf class. The tasselseed mutants fail to abort the silk in the male tassel, rendering them female; and the dwarf mutants restore perfection to the female flowers. Together these two mutations change the monoecious maize plant into a plant that bears only perfect flowers. Singly, the dwarf mutants in Zea mays and the CmACS7 mutant in Cucumis sativus are capable of changing these monoecious plants to andromonoecious plants, showing this rote of evolution of monoecy . The Cucumis sativus mutant CmWIP1 represents a change from monoecy to gynodioecy suggesting a route from monoecy to dioecy. Monoecy is able to convert to dioecy, as shown by the high number of monoecious families containing nested dioecious species but I believe it would be a mistake to consider monoecy merely a stepping stone to dioecy. Unlike dioecy, monoecy allows plants to retain their ability to be both mothers and fathers, but allows specialisation of floral forms that is so crucial for wind pollinated plants. From a human perspective, monoecy has been a powerful tool in the genetic manipulability of Zea mays, allowing both hybrid crops and extensive genetic research in the modern era .As we have seen, gyno- and androdioecy lead towards dioecy. When plants in a population become sex specific, there is an evolutionary pressure for hermaphrodites to specialise in the other direction thereby producing a dioecious population. Dioecy can also evolve from monoecy, as predicted by dioecious species nested within monoecious evolutionary groupings9 . Simple dioecy is not the end of the road – it can become more and more fixed and derived as evolutionary time proceeds. It is thought that at first dioecy is characterised by simple sex determining regions in the genome. These will often be clustered on a single autosome as is the case in Populus balsamifera. In this species, an SDR has been described on chromosome 1917. This region includes 13 genes, one of which is a paralogue of methyltransferase1 . Another gene within the region is a cytokinin signaling gene PbRR9 and methylation of this gene is predictive of sex of the tree21. P. balsamifera is therefore a less derived dioecious species which still relies on epigenetic modifications for its sex determination loci that reside on autosomes.