Cannabis spp. on the other hand has true sex chromosomes that exist as a heterochromatic pair where XX defines female and XY defines male. Cannabis was known in the literature to be under XY control as early as 1927 when female plants were damaged by having branches removed at an early stage of blooming, giving rise to male flowers, allowing the plants to be selfed. The selfed female intersexed plants gave rise to all female progeny. Similar techniques are still used today to give feminised seed. The fact that simple mutilation gives rise to the development of male flowers on genetically female plants shows that each plant has the genetic capacity to produce either sex, while the XY chromosomes dictate which pathways are activated. Interestingly, damage activates maleness, perhaps because a damaged plant producing pollen still has a chance of reproduction, while a damaged plant is not likely to be able to produce healthy seed, which is costlier than pollen. Diverting to maleness can thus be thought of as an insurance policy. The story of dioecy does not end with the evolution of sex chromosomes. Urtica dioica has an SDR, but while the name U. dioica implies dioecy, monoecious plants have been reported in the Netherlands and other parts of Europe. Further, U. dioica does not always follow a 1:1 male:female ratio in its naturally occurring populations. Thus,vertical grow cannabis designs it seems clear that U. dioica does not follow a simple XX/XY or ZW/ZZ mode of sex determination. Self-pollination of monoecious U. dioica individuals yielded a 1:2:1 ratio of male:monoecious:female individuals. This is consistent with a bi-allelic model where heterozygotes are monoecious and the two homozygotes yield male or female plants.
When a true female plant is crossed with a monoecious plant, a 1:3 monoecious:female ratio is observed in the offspring. If the female were homozygous and the monoecious were heterogametic, we would expect a 1:1 monoecious to female ratio; instead partial dominance of the dioecious female allele is proposed, leading to the AMaD plants being half female and half monoecious. When a male plant is crossed to a monoecious plant, a 3:1 male:female ratio is produced. This is consistent with the male plant being heterozygous. Further, crosses between male and female plants did not yield simple 1:1 male:female ratios, suggesting the existence of sex selection biases at some point in the developmental process. Thus, dioecy is not the end of the road, and return to monoecy can occur by surprising routes .Not only has almost every plant hormone been implicated in sex determination in plants, but their roles are often not consistent across plants. While gibberellic acid promotes female inflorescence development in Zea mays and Cucumis spp., applications of GA are capable of masculinising female Cannabis plants. Blocking brass inosteroids in the monoecious Cucurbita pepo increased male flower production, while BR knockouts in maize are feminised. Similarly, while jasmonic acid is involved in stress induced male sterility in Arabidopsis thaliana and Oryza spp., it plays a vital role in gynoecium abortion in Zea mays to allow flowers to develop as male. Other hormones are more conserved in their function. Ethylene has been shown to be involved in gynoecium development in the perfect flowers produced byA. thaliana and tobacco and has been exhaustively shown to be involved in female flower development in the Cucurbitaceae as well as being implicated in female flower development in Cannabis spp.. Abscisic acid promotes female flower development in Cucumis spp.and has also been shown to block antheridium development in ferns. Cytokinin is involved in anther development, and Z. mays ZmCKX1 cytokinin biosynthetic mutants develop as male sterile, while in Populus balsamifera, methylation of a the cytokinin signalling gene PbRR9 is associated with female trees.
In sum, there are two sets of sex determining hormones in plants. One set is directly involved in sex organ development, and this includes ethylene and ABA, which promote femaleness; while cytokinins represent a conserved male promoting hormone. Likely the roles of these hormones are ancient, and they provide avenues for the evolution of unisexual flowers which as we have seen occurs sporadically and frequently across angiosperms. Another set includes JA, BR and GA. These hormones are involved in growth, cell cycle regulation, cell death and cell expansion, therefore they can be co-opted to produce sex determination pathways in unisexual plants by turning on and off appropriate organ growth. Since they are not historically specific to male or female organ development, their roles as masculinising or feminising hormones are not consistent across angiosperms. The fact that some hormones are ancient and conserved regulators of sex expression and others are non-specific underlines the facts that the flowers of the common ancestor of angiosperms was perfect, but they evolved from the imperfect gymnosperms.Sex determination in plants is a highly complex and varied affair. As we have seen, unisexuality has evolved multiple times in angiosperms and thus the hormonal control of this process is partially inconsistent, though consistencies exist because certain hormones are inherently tied to male or female sex organ development. Sex determination is also a highly plastic process in plants; and species that humans call “dioecious” have been observed to produce different floral forms depending on the environment. Further, single mutants can completely disrupt the monoecious or dioecious habit of plants, showing that sex determination in plants is highly labile over evolutionarily time. The presence of single mutants in maize that drastically affect sex determination pathways highlights the plasticity of sex in maize and other plants. My analysis of the pleiotropic mutant fun further supports this view of sex and gender fluidity that is observed in plants and animals, calling into question the traditional binary view of sex.Unlike 95% of angiosperms, Zea mays is monoecious – having separate male and female flowers or inflorescences on the same plant.
The male inflorescence is found at the apex of the corn plant, being the final fate of the apical meristem, and is known as the tassel. From a functional perspective, this placement makes sense as it allows the pollen from these flowers to be efficiently dispersed by the wind. The female inflorescence – or inflorescences for often there are multiple on the same plant – is known as the ear. Ears develop from axillary buds, and are found around the 5th-7th leaf . Along with the obvious differences between these unisexual flowers – the male flowers lack carpels, and the female flowers lack stamens – there are a number of other differences between these inflorescences. The main stem of the ear is noticeably thicker than that of the tassel,vertical grow dry racks likely due to the fact that ears support kernels that are much heavier than the male flowers of the tassel; similarly the internodes subtending the ear are very short and compact, lending further support to this heavy inflorescence. A tassel, while slim, is much longer than an average ear; some of my own measurements from this study exceed half a metre for wild-type tassels – a quarter of the total height of the plant! Spikelets in the tassel bear two florets, while in the ear, the lower floret aborts. Finally a tassel bears secondary and sometimes tertiary branches from its main stem while a normal ear completely lacks branches. The maize leaf is a fairly typical grass leaf. It has two main parts – the blade and the sheath, separated by the ligule/auricle region. The sheath is the part closest to the stem and wraps around the stem, providing support for the long blade. The ligule, a thin epidermally derived fringe of tissue, exists on the adaxial side at the distal end of the sheath, and perhaps contributes to stemming the flow of water that could carry pathogens into the space between the stem and sheath. The auricle is immediately distal to the ligule consisting of two wedges of tissue that compensate for the angle at which the blade sticks out from the stem and sheath. Finally the blade, the main photosynthetic organ of the maize plant, extends from these basal supporting organs. The maize leaf is further described in Chapter 2.The fun tassel has multiple traits that make it resemble a feminised inflorescence, or, an ear. Just as ears are branchless and shorter than tassels, the fun tassel makes half the number of branches as compared to normal siblings and is on average half the length of normal siblings .
Shortening can also be seen in the four internodes directly subtending the tassel . Since the ear shank resembles a shortened branch made up of very compact internodes, a reasonable hypothesis would be that the feminisation of the tassel in fun results in a signal that causes the subtending internodes to behave as if they are supporting an ear. In support of this hypothesis, the lengths of internodes 5 and below are indistinguishable from normal siblings, implying that only those close to the feminised tassel are affected. Flowers in the tassel and ear initiate both stamen and carpel primordia, but male flowers abort the carpel and female flowers arrest stamen growth. The feminisation of the fun tassel flowers appears to arise from an early lack of abortion of the carpel and arrest of stamens, much in the way the flowers in the ear develop in normal maize plants . Additionally, ears in fun plants progress slower than normal siblings and are displaced lower on the plant . The fun ear is smaller than normal siblings . When considering the feminisation of the fun tassel, it is also important to note that it differs from the classic feminisation mutants of tasselseed1 and ts2. These mutants merely feminise the flowers of the tassel, leaving the branch number, architecture and floret number intact . As a result, the ts mutant apical inflorescences bend over from the weight of their heavier flowers. In contrast, the fun mutant inflorescence is able to maintain the erect form associated with a tassel since it is shortened and has fewer, shorter branches and a thickened rachis to make it more robust and able to support a feminised habit. Further, the fun feminised tassel flowers abort the lower floret, while in the ts mutants, both florets develop despite the feminisation of the tassel . When comparing these two mutants, it is possible to appreciate how fun is more globally feminised while ts1;2 are more superficially feminised.Although adult fun leaves lack an auricle, they retain an obvious line of white tissue over the ligule region that can be observed on the abaxial side of the leaf . fun mutants have a leaf angle that is on average 10° steeper in the mutant than in normal siblings. Sometimes flaps of true auricle tissue appear at the margins of the leaf, but they do not seem to be associated with any bending out of the blade and instead fold outward implying that the upright habit of fun leaves is controlled by the distortions at the midrib. The ligule itself is unaffected , but the region overall is subjected to a host of distortions as compared to normal siblings. At the midrib, where the blade intersects with the ligule boundary , a pronounced bump is observed in the mutant . Transects of the blade 1 cm above the ligule boundary reveal a change in shape of the midrib region from a tight triangular shape in normal leaves, to a broadened horseshoe shape that invades the blade tissue adjacent to it in mutants . The clear cells of the midrib that appear as a white pith to the naked eye are also spread out in this horseshoe shape within the mutant midrib. The correlation of a lack of auricle with this broadening of the midrib may imply antagonistic signalling between these two tissue types as further up the leaf the midrib appears more like wild type. There is an overall distortion in the shape of the plant, presumably due to these auricle distortions, with the whole plant often exhibiting a curving habit not seen in wild type . Though the following observations have not been quantified, I include them for completeness: the fun leaves often appear to be a darker green than normal siblings, and the whole plant seems to senesce earlier at the end of the season.