J F M A M J Jasondj F M A M J Jasond

Figure 5.4. The time of reproductive development, fruit dehiscence, and seed germination with respect to drought and rainfall periods for Opuntia and Stenocereus spp. Adapted from Pimienta-Barrios (1999).

propagation through vegetative parts is common for them in both natural and cultivated populations in arid regions (Grant and Grant 1971; Nava et al. 1981; Pimienta-Barrios 1990; Mandujano et al. 1996, 1998; Negrón-Ortiz 1998). Vegetative apomixis is particularly important, because the propagules carry reserves of water and energy, allowing them to withstand prolonged drought. Indeed, certain species of this genus, especially cylindropuntias, reproduce exclusively or nearly exclusively asexually.

One disadvantage of asexual reproduction can be its low dispersion. In many species of Opuntia, however, vegetative apomixis can be highly successful, in part due to its high dispersability. The cladodes of platyopuntias and the joints of cylindropuntias can be readily detached and adhere by means of the spines to the skin of cattle, wild life, and other animals that touch the plants. This favors the invasion of opuntias in overgrazed areas (Anthony 1954). Humans also play an important role in spreading opuntias vegetatively. In semiarid areas of central Mexico, shepherds and cattle ranchers commonly cut off the cladodes of opuntias for animal feeding or to have easy access when walking. Some of the cladodes, or even their fragments, may root and give rise to new individuals. Indeed, nopaleras, a vegetation type in central Mexico dominated by Opuntia spp., may be anthropogenic (González-Espinosa 1999). Vegetative apomixis can cause opuntias to become noxious weeds. The epibiotic outbreak of Opuntia stricta in Australia after its introduction in 1839 is the best example. In 1920 this species occupied 24 x 106 hectares and was colonizing about 400,000 hectares per year. The moth Cactoblastis cactorum was brought from Argentina to control the outbreak (Harper 1977).

Some asexual species of Opuntia provide the best examples of high dispersal rates among cacti. Opuntia frag-ilis, for instance, is a low plant, 5 to 10 cm tall, with an enormous geographic range from Chihuahua and Coa-huila in Mexico (Bravo-Hollis 1978) up to Saskatchewan, Alberta, and British Columbia in Canada, including 16 states in the United States (Mitch 1970). Its altitudinal range is from sea level up to 2,400 m in a great variety of vegetation types. Flowering and fruit production are rare, and fruits are usually sterile (Benson 1982; Mitch 1970). Bison probably helped to disperse it (Mitch 1970). Another example is O. polyacantha, which has sterile fruits, high phenotypic diversity, and a wide geographic distribution from Chihuahua and Coahuila, Mexico, to Canada (Benson 1982; Mitch 1970).

In certain species of Opuntia, sexual reproduction occurs only sporadically. For instance, despite its high seed production, most of the population regeneration of O. rastrera is through asexual reproduction (Mandujano et al. 1996), although in grassy areas of its range, O. rastrera reproduces mainly sexually (Mandujano et al. 1998). Germination rates for various Opuntia spp. are usually low, at least soon after the seed is released (Trujillo and González-Espinosa 1991). Germination rates may increase steadily with aging (Mandujano et al. 1997). However, predation rates in natural habitats may also be high, particularly by rodents (González-Espinosa and Quintana-Ascencio 1986). Thus, mutations favoring asexual reproduction may be common. Indeed, asexual clones can be derived from sexual species. Opuntia ficus-indica produces normal seeds, but sterile clones occur in which mutation back to sexual reproduction is possible (Weiss et al. 1993).

Whereas vegetative apomixis is widespread among opuntias and may contribute to their large geographic range, it is less important in the rest of the Cactaceae. For vegetative apomixis to be an efficient means of dispersal, the plants must have small detachable units, as happens for small opuntias, such as O. fragilis. In many opuntias with large cladodes, vegetative apomixis may not be an efficient way of reproduction, because sooner or later the limited dispersal would cause the saturation of suitable available habitat (cf. the strawberry-coral model; Williams 1975). In certain species of Opuntia, however, humans can circumvent this problem by promoting vegetative reproduction artificially, as may have happened for the Mexican nopaleras. Other cactus genera, such as Hylocereus, can be propagated vegetatively (Ortiz-Hernández 1999). However, reproduction by seed is the most common reproductive method for most cacti.

For certain plants, normal seed is set but sexual fusion does not occur. Compared to vegetative apomixis, agamo-spermy has the advantage of seminifery, because the seed is a vehicle of dispersal that confers resistance to environmental extremes, such as through dormancy (Heslop-Harrison 1983). As for vegetative apomixis, agamospermy is also a common asexual reproduction method for species of Opuntia (Ganong 1898; Archibald 1939; Tiagi 1954; Flores and Engleman 1973; García-Aguilar and Pimienta-Barrios 1996; Négron-Ortiz 1998), as well as Mammillaria (Ross 1981). Polyembryonic seeds are common in most of the wild and cultivated Opuntia spp. growing in semiarid central Mexico (Trujillo and González-Espinosa 1982;

Figure 5.5. Multiple proembryos in the embryos sac of the polyembryonic Opuntia streptacantha, showing a laminar zygote proembryo (z) and globular adventitious proembryos (g). Scale bar = 50 pm.

Pérez 1993). For example, seeds of many wild opuntias have two and, on rare occasions, three embryos (Trujillo and González-Espinosa 1982), and the percentages of polyembryonic seeds varies from 0.5 to 20% (Pérez 1993). The production of both sexual and asexual seeds may be considered a facultative apomixis, exchanging genetic material by occasionally producing sexual material and achieving a balance between stability and flexibility that permits adaptation to stressful environments.

Mammillaria prolifera is apomictic by adventitious embryos. However, some primarily outcrossing (allogamous) taxa, e.g., Mammillaria tenuis and M. zeilmanniana, are partially apomictic by adventitious embryos after endosperm formation. Self-sterile polyploids of M. compressa, M. parkinsona, and Gymnocalccium brunchii have extensive vegetative branching. Opuntia spp., which have a high frequency of vegetative propagation, adventitious embryos, and self-fertility, have extensive polyploidy (Ross 1981).

Embryos from apomictic seeds generally originate from nuclear tissue, and nuclear embryogenesis commonly occurs without pollination (Tisserat et al. 1979). However, a comparative study of embryo-sac development in a monoembryonic/polyembryonic Opuntia species to assess the cytological origin of both sexual and agamospermic embryos reveals that, in the polyembrionic species, multiple embryos are differentiated in the central cell of the embryo sac at the mycropilar side; however, pollen tubes do not penetrate the mycropile, indicating that fertilization does not occur (García-Aguilar and Pimienta-Barrios 1986). The embryos located at the periphery of the embryo sac are globular and without a suspensor, differing from a well-developed laminar embryo with a well-defined suspensor located at the mycropilar side of the embryo sac (Fig. 5.5). Because of their position in the embryo sac and their morphology, the laminar embryo with its suspensor apparently differentiates from the egg cell, and the globular embryos without suspensors derive from the nuclear cells and further invade the embryo sac (Bhojwani and Bhatnagar 1979; Tisserat et al. 1979). The lack of evidence of meiotic chromosome reduction during megasporogen-esis and fertilization in the polyembryonic species suggests that the laminar embryo, with a well-defined suspensor, develops by diplospory-parthenogenesis (gametophytic apomixis), and the globular embryos without suspensors develop by adventitious embryony (Heslop-Harrison 1983).

Conclusions

The reproductive systems of cacti include such striking features as combining of two or more reproductive methods and switching between reproductive modes during the lifetime of an individual. Asexual reproduction can fix favorable combinations of genes, which were produced previously by sexual reproduction. Selfing can be an economic way of sexual reproduction and also functions as a genetic barrier, preventing or reducing hybridization and permitting the production of seed when external pollination is unreliable. Outcrossing is undoubtedly a genetic method favoring the wide diversity observed for cacti. But all of these forms of reproduction have genetic costs (e.g., seg-regational load with outcrossing, inbreeding depression with selfing, and probably mutational load with asexual reproduction) and ecophysiological costs to the plant (e.g., in a particular outcrossing, attracting pollinators and resource demands). In some cases, the partial or complete suppression of one of these reproductive systems has been successful, as for species with unisexual individuals or the sterile or partially sterile opuntias.

The reproductive versatility is extremely widespread in members of the genus Opuntia, and it can play an important role in the ecological strategy of adaptation to aridity. Not surprisingly, Opuntia is the most widely distributed genus in the Cactaceae. Members of this genus have self-pollinating as well as cross-pollinating flowers. Both sexual reproduction and asexual reproduction by vegetative parts and seeds occur. These versatile reproductive systems are often controlled by environmental factors, suggesting that the genetic systems may exhibit phenotypic plasticity. Although asexual reproduction can be very successful, in some circumstances sexual reproduction, or the mixed mating system (most likely the ancestral form) with its combination of outcrossing and inbreeding, predominates. The evolutionary importance of sexual reproduction is reflected by its existence, persistence, ubiquity, and presumably lower energy investment, despite the great potential among cacti for asexual reproduction.

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