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The African oil palm Elaeis guineensis is a monoecious species of the palm subfamily Arecoideae. In particular, the enhancement of male inflorescence production in response to water stress has been well documented. This paper presents a review of our current understanding of the sex determination process in oil palm and discusses possible insights that can be gained from other species.
Although some informative phenological studies have been carried out, nothing is as yet known about the genetic basis of sex determination in oil palm, nor the mechanisms by which this process is regulated. Nevertheless new genomics-based techniques, when combined with field studies and biochemical and molecular cytological-based approaches, should provide a new understanding of the complex processes governing oil palm sex determination in the foreseeable future.
Current hypotheses and Sex text palm for future research are discussed. The physical separation of fertile male and female reproductive organs on different flowers of the same plant or different individuals of the same species is a condition known as dicliny.
Diclinous species are estimated to represent approx. Dicliny can be divided into two main : monoecy, in which male staminate and female pistillate flowers are produced on the same plant; and dioecy, in which single-sex male and female plants exist. Other more complex situations involving the co-occurrence of male, female and bisexual flowers or plants are collectively classified as polygamy. Although spatial separation of male and female reproductive organs is employed by many species as a means to achieve outbreeding, a similar result can be achieved by temporal separation.
In its simplest form, temporal dioecy can be achieved simply by the delayed maturation of either carpels in the case of protandrous flowers or stamens in the case of protogynous flowerstwo variants of the condition known Sex text palm dichogamy.
The palm family Arecaceae is a particularly interesting group in which to investigate dicliny and dichogamy, as both are prevalent within this clade Loo et al. As regards spatial separation of the sexes, the most common condition in palms is monoecy, which s for approx.
The majority of monoecious palms produce bisexual inflorescences, with only a handful Sex text palm exceptions, notably in the genera LepidorrachisArengaWallichiaWettiniaMarojejyaAttalea and Elaeis Voeks, ; Baker and Hutton, The African oil palm Elaeis guineensis is the best studied example of temporal dioecy in the palm family, due in particular to its economic importance as the world's largest source of vegetable oil.
Considerable efforts have been devoted over the last half century or so to improving the yield potential of cultivated palms and to perfecting the growing conditions necessary to realize this potential. In this paper, we first discuss current knowledge of the mode of action of environmental factors on oil palm inflorescence development, and then discuss how they might act upon the plant's reproductive development to regulate sex determination.
As mentioned earlier, the oil palm normally produces functionally unisexual male and female inflorescences in an alternating cycle, thereby minimizing the chances of self-pollination.
Macroscopic views of the normal male and female inflorescences are shown in Fig. Male and female inflorescences are branched and easily distinguishable from their overall structure, the male inflorescence bearing a larger of rachillae branches inserted into the rachis central axis. Another easily recognizable difference between the two sexes is in the form of the rachillae themselves; in the case of the female inflorescence they bear a distal sterile region that develops into a characteristic spine not seen in the male rachilla.
The early differentiation of oil palm inflorescences was studied by scanning electron microscopy SEM by Van Heel et al. This reflects the larger of flowers borne on the male rachilla and confirms the earlier conclusions of Corley a. A later key difference between the male and female inflorescences concerns the arrangement of flowers on the rachillae, the staminate flowers being borne singly whereas pistillate flowers are flanked by a pair of abortive accompanying staminate flowers.
The female inflorescence thus produces floral tri, a structural arrangement that is dominant although not universal in the subfamily Arecoideae. The accompanying staminate flowers develop pollen sacs with proliferating microsporocytic cells; however, the latter display incomplete development and the flower is shed soon after, before the pistillate flower of the triad has reached maturity Adam et al. Both types of inflorescence are protected during their development by the prophyll and first penduncular bract, which are ruptured just before flower maturity is reached.
Other peduncular bracts are small, triangular structures which do not envelope the developing inflorescence Dransfield et al. Macroscopic views of oil palm inflorescences: A mature female inflorescence, B mature male inflorescence. In favourable growing conditions, an inflorescence is initiated in the axil of each leaf of the palm. The rate of leaf emission varies with age, with typically three leaves produced per month in young palms and Sex text palm per month in the case of older individuals.
The development of an oil palm inflorescence between the stages of initiation and flower maturity lasts 2—3 years for both sexes, the process being initiated soon after seedling establishment. In tropical humid climates with regular rainfall, inflorescence and fruit production is spread evenly throughout the year. From an experimental point of view, this allows the possibility of dissecting mature palms so as to visualize a complete series of inflorescence developmental stages within the same individual. As a means of cross-referencing between palms, inflorescences are usually ed according to their subtending leaves.
A summary of the key stages of inflorescence development in relation to leaf is shown in Fig. Typically, a mature palm will alternate between male and female inflorescence production during its lifetime; however, the proportion of time spent Sex text palm each phase will vary considerably depending on both environmental and genetic factors. The sex ratio is normally defined as the ratio of female to total inflorescences in a given group of palms.
In regions with high and regular rainfall e. Malaysia and Indonesiaoil palm sex ratios tend to vary little throughout the year, in contrast to areas experiencing a marked dry season such as in West Africa, where the sex ratio undergoes extensive fluctuations.
Corley a noted that in the latter case, the period of lowest sex ratio high male inflorescence production occurs during the rainy season and speculated that this character is an adaptation against the reduction in airborne pollen density caused by high atmospheric humidity.
This illustrates the well-established observation Sex text palm oil palm sex determination is strongly influenced by climatic factors, with male inflorescence production being promoted by water deficit. Male inflorescence production can also be induced artificially by defoliating adult plants, a technique which has proved invaluable for increasing pollen production from pisifera palms Durand-Gasselin et al.
It is interesting to note that pisifera palms produce abortive fruit, which place lower demands on the plant's resources. As a result, pisifera palms usually remain female when grown in conditions of high and regular water availability, thus creating the problem of limited pollen production for breeding, which can be overcome by defoliation as mentioned earlier. The exact nature of the stress response underlying the induction of male inflorescence production by defoliation is not yet clear, but current hypotheses will be discussed in more detail below.
Beyond the obvious agronomic interest in using defoliation as a means of controlling pollen production, the same approach can also provide a useful experimental system by which to investigate the process of sex determination in oil palm. In the context of the studies described here, defoliation can be defined as the pruning of all expanded leaves at their bases so as to remove the entire lamina, the unopened leaves in the spear being cut around 1 m from their base Durand-Gasselin et al.
Field observations of sex ratio changes in response to defoliation have provided clear indications of the chronology of sex determination and differentiation. The timeline of oil palm inflorescence development, as inferred from the effects of defoliation, are summarized in Fig. It can be seen Sex text palm the time intervals between the different stages of inflorescence development vary according to the age of the palm, in accordance with the lower rate of leaf emission seen in mature palms compared with young individuals. One important point to note is that sex determination is not the only effect of leaf pruning; the latter treatment also induces inflorescence abortion.
As with most aspects of flowering, the exact chronology of these effects varies according to palm age. Although it can be promoted artificially by leaf pruning, inflorescence abortion is in fact a commonly observed phenomenon in oil palms. Some evidence has been obtained to suggest that inflorescence abortion is sex-dependent, by examining the effects of moderate leaf pruning Corley, In the latter study, no effect was observed on the sex of inflorescences reaching maturity during the first 3 months after pruning; however, over the following 6 months, the sex ratio was ificantly lowered.
This indicates that in the specific case of leaf-pruning-related stress at least, female inflorescences undergo a higher rate of abortion than male inflorescences. It should be borne in mind that the more extreme stress induced by pruning the laminae of all expanded leaves in the abortion of inflorescences of both sexes. Development and sex determination of the oil palm inflorescence in relation to palm age [after Hartleyas reproduced by Durand-Gasselin et al. The ability of oil palms to change the sex of the inflorescences they produce in response to environmental factors provides the basis for a useful experimental system to manipulate reproductive development in palms and study its molecular determinants.
To date, the character of sex determination per se has not been used by oil palm breeders in genetic improvement programmes, so nothing is known regarding the or type of genetic loci which might be involved in determining maleness or femaleness. Nevertheless, the selection of genotypes conferring high oil yield has inevitably been accompanied by a general increase in sex ratios, which provides indirect evidence that genetic factors also come into play. Indeed, with palms of highly selected genotypes planted in favourable growing conditions, it is commonly observed that male inflorescence production is close to zero in the first year or two of flowering, thus jeopardizing fruit production.
The fact that oil palm sex ratios may be enhanced or reduced depending on genotype, coupled with the fact that a genetic map has been produced for this species Billotte et al. The ability to vary sexual expression in response to the environment provides plants with a means of adjusting resources allocated to ovule and pollen production, thereby conferring adaptive flexibility Korpelainen, Gender plasticity has been observed in plant species belonging to a wide range of phylogenetic groups. In the case of androdioecious populations of Mercurialis annua Euphorbiaceaein which male and hermaphrodite plants coexist, progeny sex ratios were found to depend on the density of the parental plants Dorken and Pannell, This phenomenon was explained in part by the increased pollen production of hermaphrodite plants when growing at low density, leading to proportionally fewer male plants in the subsequent progeny.
Other environmental factors that have been shown to affect sex ratios in plant populations include daylength and light intensity, as illustrated by the monoecious or sometime polygamous species Atriplex halimus Chenopodiaceaewhich displays increased femaleness under short days and low light irradiance Talamali et al. In the case of the sexually dimorphic woody pioneer species Antirhea borbonica Rubiaceaea polliniferous morph was identified and studied Litrico et al.
It was found to display considerable gender plasticity, almost no fruit-bearing female flowers being produced in pioneer populations, whereas in late succession, high fruit-sets were observed although they were lower than for the female morph. In a clonal context, gender plasticity may operate not only in relation to environmental conditions but also in relation to the maturation of the plant, as seen in the andromonoecious species Pennisetum typhoides Poaceae; Sandmeier and Dajoz, or the monoecious aquatic Sagittaria sagittifolia Alismataceae; Dorken and Barrett, In the case of the subdioecious species Urtica dioica Urticaceaein which monoecious plants coexist with female and male individuals, only the former were found to display a variable sex ratio, with different clones of the same genotype varying in the percentage of male flowers produced Glawe and de Jong, Subsequent studies revealed that the maternal parent strongly contributes to sex ratios, the exact genetic mechanism involved remaining as yet unresolved Glawe and de Jong, Although relatively few data are available for the palm family, sexual expression has been shown to be influenced by environmental factors not only in oil palm but also in two different species of the monoecious Arecoid genus Attalea.
The data obtained in this study suggested that sexual expression in A. A second species of AttaleaA. In this case, the onset of reproduction and sex allocation were compared in three contrasting environments: forest, partially invaded pasture and a pure stand babassual. Reproduction was found to start at a smaller size in the pasture and pure stand compared with forest.
Male flowering started at a lower height than female flowering in the pasture and pure stand; however, this was not the case in the forest environment. Although several hypotheses were raised to explain these data, it seems probable that the earlier flowering observed in more open environments was made possible by the much greater light and therefore energy resources available Sex text palm palms in these habitats. At a more general level, although there is some scope for the optimization of resource allocation to male and female reproduction in hermaphrodite species by the modulation of pollen and ovule development, the greatest flexibility is probably afforded by the monoecious and polygamous conditions, which allow the adjustment of the relative s of specialized male and female flowers both within the same plant and between individuals.
One intriguing case is that of the Madagascan Marojejya darianii : a of specimens of this species were successfully grown to maturity on the island of Hawaii, but as yet, after several years of flowering, only male inflorescence production has been observed Marcus, Although environmental factors may play a causal role in cases such as this, it Sex text palm also be speculated that endogenous factors within the plant that change during its life-cycle progression e.
Dioecy and monoecy appear to have arisen on a large of occasions during the evolution of flowering plants, as witnessed by their wide and fragmented distribution within the Angiosperm tree of life. This suggests that the genetic changes that resulted in the monoecious and dioecious habits are different for the most part in different evolutionary lineages Ainsworth, Nevertheless, thanks to comparative genomics and evo-devo approaches, it is clear that a wide range of alling elements that regulate reproductive development are conserved throughout flowering plants.
An early indication of this conservation was Sex text palm by comparing the first two Angiosperms to have their genomes sequenced, Arabidopsis thaliana and rice Izawa et al. The latter authors carried out a survey of the structural and functional conservation of genes between the two model plants, using flowering pathways as an example.
This study revealed that a wide range of flowering-related genes are structurally conserved between monocots and eudicots. Nevertheless, functional divergence appears to have frequently occurred to give novel regulatory mechanisms which distinguish the two model species, this degree of divergence providing a glimpse of the situation in flowering plants as a whole.
As regards sex determination, although it seems likely that a generalized alling cascade for flower development has been partially maintained during evolution, the genetic basis of sexual dimorphism Sex text palm se is likely to vary considerably. From a developmental point of view, this hypothesis is supported by the great variety of ways in which unisexual flower development is achieved.
Examples of the latter, provided by the palm family, include non-initiation of the undesired organ e. As discussed by Weiblen et al. Although approx. Genome sequences have already been obtained for the dioecious species poplar Tuskan et al. In the latter case a small male-specific region MSY was identified on the Y chromosome and it was proposed that two sex determination genes controlled the sex determination: a feminizing or stamen suppressor gene causing stamen abortion before or at flower inception; and a carpel suppressor gene causing carpel abortion later in flower development Ming et al.
For this species, a study Sex text palm made of the phenotypic effects of X-ray-induced deletions Lebel-Hardenack et al. This approach provided data to confirm the classical genetic prediction of three dispersed male-determining loci on the Y chromosome, one for carpel suppression, one Sex text palm early stamen development and another for late stamen development.
A similar approach was used by Koizumi et al. In this case C-ion beam radiation was used to obtain a Y chromosome lesion that resulted in the production of hermaphrodite flowers with a mature gynoecium. Although the identity of the gene or genes responsible for each of these activities remains to be determined, complementary studies of sex-specific expression have enabled the identification of a of genes of interest Robertson et al.
Two plant species for which sex determination gene functions have been clearly attributed are melon and maize. In the case of the polygamous species melon Cucumis melosex ratios are controlled by allelic compositions at two different loci. Boualem et al. Martin et al. Expression of CmWIP1which was abolished in the transposon insertion mutant, led to carpel abortion and also indirectly repressed the expression of CmACS-7 to allow stamen development.
Collectively, these data provide an interesting insight into the sex determination process in the Cucurbitaceae and also reveal some of the genetic basis of hormone-dependent sex determination, a common phenomenon in diclinous plant species. Another functionally characterized model is that of the monoecious species maize, which produces morphologically distinct male and female inflorescences on the same plant. For this species, a of tasselseed mutants displaying feminized male flowers have been isolated and characterized and a alling pathway has emerged Calderon-Urrea and Dellaporta, These two genes were cloned and found to encode, respectively, a lipoxygenase involved in jasmonic acid synthesis and a steroid dehydrogenase DeLong et al.Sex text palm
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