Living together and living apart: the sexual lives of bryophytes

Abstract

One contribution of 15 to a theme issue 'Weird sex: the underappreciated diversity of sexual reproduction'. Haploid gametophytes of bryophytes spread by clonal growth but mate locally, within an area defined by the range of sperm movement. Rarity of establishment from spores or vegetative competition can result in unisexual populations unable to reproduce sexually. Females typically outcompete males, probably because females expend fewer resources than males on the production of gametes. Extreme sexual dimorphism-tiny males growing as epiphytes on much larger females-has evolved many times. Haploid selfing is common in bryophytes with bisexual gametophytes, and results in completely homozygous sporophytes. Spores from these sporophytes recapitulate the genotype of their single haploid parent. This process can be considered analogous to 'asexual' reproduction with 'sexual' reproduction occurring after rare outcrossing between haploid parents. Ferns also produce bisexual haploid gametophytes but, unlike bryophytes, haploid outcrossing predominates over haploid selfing. This difference is probably related to clonal growth and vegetative competition occurring in the haploid but not the diploid phase in bryophytes, but the reverse in ferns. Ferns are thereby subject to stronger inbreeding depression than bryophytes.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'. Sexual lives of bryophytesAnimal life histories are characterized by multicellular diploid individuals that produce haploid gametes by meiosis. Gametes are the only haploid cells of such life cycles. The life histories of bryophytes (mosses, liverworts and hornworts) are fundamentally different. Multicellular haploid gametophytes (gameteproducing plants) produce gametes by mitosis. The fertilization of an ovum by a sperm produces a zygote that develops into a short-lived multicellular diploid sporophyte (spore-producing plant) that grows attached to the maternal gametophyte on which it depends for nutrients. Sporophytes produce haploid spores by meiosis that germinate to produce new gametophytes (figure 1). Mating is restricted to the distance over which sperm can find ova, usually in the millimetre to centimetre range [1]. Clonal expansion occurs via vegetative spread of gametophytes into neighbouring space and production of gemmae (asexual propagules) that disperse gametophytic clones to new sites [2].Gametophytes of dioecious bryophytes are unisexual, producing either eggs or sperm, but not both. Males and females compete for space but require close proximity for sexual reproduction. By contrast, gametophytes of monoecious bryophytes are bisexual, able to produce both eggs and sperm. A bisexual gametophyte that fertilizes its own eggs engenders homozygous sporophytes all of whose spores are genetically identical (excepting meiotic errors and postzygotic mutations). Roughly 70% of liverworts, 60% of mosses and 40% of hornworts are dioecious [3]. Bryophyte life cycles are not only fascinating in th...

Recommendations for the use of critical terms when applying IUCN red-listing criteria to bryophytes

“…In bryophytes, single ramets may reproduce asexually, and even sexually in the case of monoicous species (Haig 2016 For some bryophytes (the minority) it may be possible to identify 'mature individuals' as suggested in the Red List Guidelines (IUCN SPSC 2017) for unitary organisms. For example, if a species forms isolated, discrete cushions, as occasionally happens in some mosses (e.g.…”

Ecological and physiological peculiarities of bryophytes on a post-technogenic salinized territory

“…By inhabiting such disturbed territories, pioneer moss turfs form a new succession with changes in humidity (Shcherbachenko et al, 2015;Rabyk et al, 2017), mineral (Vilmundardóttir et al, 2018) and organic status of the substrate (Karpinets et al, 2016;Kyyak & Baik, 2016;Karpinets et al, 2017). The participation of bryophytes in the revitalization of plant cover of technogenic ecosystems is determined by their high tolerance to drying (Kyyak & Khorkavtsiv, 2015;Kyyak et al, 2017;, their ability to restore soil due to the structuring of its upper horizons (Carter & Arocena, 2000;Aronson & Alexander, 2013;Jackson, 2015), to prevent its erosion (Haig, 2016;Baughman et al, 2017;Stark, 2017), to absorb and retain moisture (Seitz et al, 2017;Batista et al, 2018;Delgado-Baquerizo et al, 2018), thereby reducing surface runoff (Greenwood & Stark, 2014;Zhao et al, 2014;García et al, 2016). Due to its specific properties of metabolism, moss turf has a significant effect on the soil chemical reaction, accelerating the exchange of cations in the biogeochemical cycle, affecting the circulation of organic carbon and nutrients through the release of mineral and organic compounds into soil solutions (Douma et al, 2007;Kyyak & Baik, 2016), synthesis of phenolic substances with a wide spectrum of antimicrobial action, which promotes the development of a substantial litter layer, in which the processes of mineralization are much slower, biogenic elements are accumulated and favourable conditions for the growth of the underground organs of vascu-lar plants are created (Cortina-Segarra et al, 2016;Bueno de Mesquita et al, 2017).…”

Weird sex: the underappreciated diversity of sexual reproduction

“…Specialist terminology is often necessary to precisely describe non-conventional organisms and behaviours, and as an unfortunate consequence, papers brimming with unfamiliar terminology are not easily seen as relevant to the study of sex, sexual selection or sexual conflict, in general. Try explaining how a female can mate with her male offspring whose dad was the male offspring of the male's mate [8]. At the same time, researchers sometimes use the same terminology but mean fundamentally different things.…”

“…The topics addressed in this special issue range from very fundamental questions such as the evolutionary benefits [12] and the origin of sex [3,4,13] and mating types [6], to secondary consequences of sexual reproduction, such as sexual selection [5,11] and the increased scope for genetic conflict [8,10,14], and the cost of sex, either due to the production of males in anisogamous species or due to mate finding or mechanisms of meiosis [15].…”

“…Schwander & Oldroyd [14] discuss an extreme example of this, androgenesis, where males exploit the female gamete and eliminate the female's genome to transmit their own genome only. Haig [8] addresses potential conflicts between male and female haploid 'interests' in mosses and ferns, which may be played out both before and after gamete fusion, and trade-offs between sexual and asexual fertility. In mosses, a haploid gametophyte produces eggs that get fertilized by sperm produced by the same or a different gametophyte to produce a diploid sporophyte.…”