monoicous

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 mon(o)ecious are derived from the Greek mόνος (mónos), single, and οἶκος (oîkos) or οἰκία (oikía), house. The words dioicous and di(o)ecious are derived from οἶκος or οἰκία and δι- (di-), twice, double. (o)e is the Latin way of transliterating Greek οι, whereas oi is a more straightforward modern way. Generally, the terms "monoicous" and "dioicous" have been restricted to description of haploid sexuality (gametophytic sexuality), and are thus used primarily to describe bryophytes. Meanwhile, "monoecious" and "dioecious" are used to describe diploid sexuality (sporophytic sexuality), and thus are used to describe tracheophytes.^[2]

In zoology, the term hermaphrodite is typically used instead of "monoecious". There are exceptions to this rule, however: for example, annelids are often described as "monoecious" 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 determination^[3] 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, the gymnosperms and angiosperms, the sporophyte phase is dominant and the gametophyte phase is both dimunitive and attached to the parent sporophyte. The sporophytes of seed plants therefore exert control over the sexuality of the gametophytes, which are always unisexual. This contrasts with bryophytes and pteridophytes, whose gametophytes may be unisexual or 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 can be both selective advantages and selective disadvantages for organisms that are monoicous or dioicous. Monoicous bryophytes can easily reproduce sexually, since both sexes can be found on the same organism. On the other hand, this can lead to inbreeding and reduce genetic variation within populations. Dioicous organisms necessarily exchange genes with other organisms of the species during sexual reproduction, increasing heterozygosity and variability (given a sufficiently large variable mating population). If isolated, however, organisms may only reproduce asexually, which could present a severe selective disadvantage over time. Bryophyte sperm dispersal can therefore be key to species longevity, particularly in dioicous species. While sperm dispersal is typically passive, with sperm dispersing through water, certain species exhibit very active dispersal mechanisms, such as aerial dispersal recently described in the liverwort Conocephalum conicum.^[4]

References