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Monoicous organisms are defined as having both sperm-producing and egg-producing reproductive organs in the same individual. By contrast dioicous organisms produce male and female reproductive organs on different individuals.¹

Etymology and history

The word monoicous and the related forms monoecious and monocous are derived from the Greek roots mono, (= single) and oikos, (= house). Similarly, dioicous and the forms dioecious and diocous are derived from the Greek di (= twice or double) + oikos, (= house). Historically, the terms "monoecious (dioecious)" and "monoicous (dioicous)" have been used interchangeably in botany, but there is a tendency to restrict monoecious and dioecious only to seed plants, referring to whether or not an individual sporophyte plant bears one or both kinds of gametophyte. Monoicous and dioicous refer to whether or not an individual gametophyte plant bears one or both kinds of gametangia.

In zoology, the preferred terminology has become hermaphrodite, rather than "monoecious". An exception are lower animals, e.g., the phylum of annelids (that covers worms and leeches): they may be monoecious (the same animal bears both ova and sperm) or dioecious.

Bryophyte sexuality

In all land plants, the haploid gametophytes are the only structures that produce gametes, and thus sexuality is fundamentally the same in all groups. However, complications arise from differences in the timing of sex determination² and differences in the relative development and importance of gametophytes and sporophytes in different plant groups.

Bryophytes have life-cycles that are generally gametophyte-orientated; that is, the normal, dominant autotrophic plant is the haploid gametophyte. The sporophyte in bryophytes is dependent, parasitic on the gametophyte, and is a reduced diploid structure consisting only of a stalked sporangium in season. As a result, in bryophytes sexuality is usually determined by the gametophyte. There are two basic categories of sexuality in bryophytes:

• Monoicous bryophytes produce both antheridia (male organs) and archegonia (female organs) on the same plant body.
• Dioicous bryophytes produce only antheridia or archegonia on a single plant body.

There are several specialized forms of the monoicous condition, each with its own terminology:

• Autoicous bryophytes are monoicous, but with the antheridia and archegonia produced in separate clusters called inflorescences. A cladautoicous plant bears these inflorescences on separate branches.
• Synoicous bryophytes produce their antheridia and archegonia together in the same inflorescence. This condition is also called androgynous.
• Heteroicous bryophyte species may be either monoicous or sequentially dioicous depending on environmental conditions. This condition is also called polygamous or polyoicous. Other species grow exclusively with one type of sexuality.

Seed plant sexuality

In seed plants, gymnosperms and angiosperms, the sporophyte phase is dominant and the gametophytes are diminutive, strongly reduced in size and complexity, developing within sporophyte tissues and completely dependent on the sporophyte for nurture, totally reversing the situation in bryophytes. The sporophytes of seed plants therefore exert control over the sexuality of the gametophytes, which by contrast with bryophytes are always unisexual, never bisexual. In seed plants but not free-sporing pteridophytes or bryophytes a monoecious plant produces male and female gametophytes in the same sporophyte, in contrast to dioecious plants, in which a single plant may have only either male or female organs. See Sexual reproduction of plants.

• Monoecious - having unisexual flowers, conifer cones, or functionally equivalent structures of both sexes appearing on the same plant;
• Dioecious - having unisexual flowers, conifer cones, or functionally equivalent structures occurring on different individuals;
• Because many dioecious conifers show a tendency towards monoecy (that is, a female plant may sometimes produce small numbers of male cones or vice versa), these species are termed subdioecious.

Role in survival

There are both advantages and disadvantages in being monoicous or dioicous. Monoicous organisms benefit because they are almost always capable of reproducing, since there is no need to find a partner of another gender. However, dependence on inbreeding increases homozygosity and reduces genetic variability in populations, which become less well adapted to survive in changing or spatially patchy environments.

Dioicous organisms, on the other hand, have the benefit of exchanging genes with other members of the species, increasing heterozygosity and variability, and thus promoting natural selection from among a wider range of desirable traits and evolution. However, they are at a disadvantage in areas of low population, where there is a lower probability of encountering a breeding partner of the opposite gender. When only one gender of a species remains, it may be considered extinct and the only possible ways to save the species is through cloning or interbreeding with a closely related species, producing a hybrid. If populations reach critically low numbers ("Bottleneck"), inbreeding may increase due to the reduced pool of possible mates, reducing the number of available allelles for evolution, and in certain species this may result in an inbreeding depression effect, where reproduction is reduced by infertility, incompatibility, or increased numbers of developmental abnormalities.

References