Non-flowering plants have rather complex reproductive life cycles compared to the more familiar pattern of fertilization and seed formation seen in Angiosperms. Sporophytes of mosses, liverworts and hornworts are often described as parasites on the gametophytes to which they are attached and from which they are nourished. But the gametophyte and the sporophyte are two stages of a single life history that undergoes coordinated growth, whereas parasitism is an antagonistic interaction between individuals of different species. Whatever label is used, relations between diploid progeny (sporophytes) and haploid progenitors (gametophytes) are expected to be neither perfectly harmonious nor purely antagonistic whenever sporophytes possess paternal alleles that are absent in maternal gametophytes. Sporophytic fitness will often be maximized by transfer of more resources than maximizes maternal gametophytic fitness.
There are two ways to conceptualize genetic individuals (genets) when a sporophyte grows attached to a gametophyte. The more familiar is to recognize the two generations as distinct individuals. The less familiar is to recognize the maternal haploid genet as extending across the gametophyte–sporophyte boundary into the sporophyte. In the orthodox account, the boundary between genets separates diploid from haploid tissues. In the heterodox account, two haploid genets (mum and dad) are physically fused in the sporophyte but nevertheless maintain distinct genetic interests. All mum’s genes are present in both generations and benefit from the same outcomes whether a particular gene is expressed in the gametophyte or sporophyte. By contrast, paternal genes are absent from mum and, for this reason, are subject to different selective forces from those experienced by maternal genes. The most recent common ancestor of mosses is much older than the most recent common ancestor of angiosperms. By this criterion, mosses encompass much greater phylogenetic diversity than flowering plants, including substantial variation in relations between gametophytes and sporophytes.
A recent paper in Annals of Botany predicts that moss sporophytes have evolved to take more nutrients from maternal gametophytes than maternal gametophytes have evolved to supply, resulting in ongoing evolutionary conflict. If transpiration has a major role in sporophytic nutrition, then sporophytes should possess adaptations to increase transpiration and maternal gametophytes adaptations to reduce transpiration.
Dioicous mosses produce unisexual gametophytes, either male or female, whereas monoicous mosses produce bisexual gametophytes. When a bisexual gametophyte fertilizes itself, a sporophyte’s dad is also its mum. The sporophyte and its single haploid parent are genetically identical at all loci, except that each locus is present in double dose in the sporophyte. Therefore, the genetic interests of maternal and paternal genes will converge as the frequency of gametophytic selfing increases and the degree of conflict will correspondingly diminish. Other things being equal, sporophytes of monoicous mosses are predicted to have shorter setae, smaller capsules, fewer and smaller stomata, and to be less profligate in their use of water than the sporophytes of dioicous mosses.
Filial mistletoes: the functional morphology of moss sporophytes. Ann Bot (2013) 111 (3): 337-345. doi: 10.1093/aob/mcs295
A moss sporophyte inherits a haploid set of genes from the maternal gametophyte to which it is attached and another haploid set of genes from a paternal gametophyte. Evolutionary conflict is expected between genes of maternal and paternal origin that will be expressed as adaptations of sporophytes to extract additional resources from maternal gametophytes and adaptations of maternal gametophytes to restrain sporophytic demands. The seta and stomata of peristomate mosses are interpreted as sporophytic devices for increasing nutrient transfer. The seta connects the foot, where nutrients are absorbed, to the developing capsule, where nutrients are needed for sporogenesis. Its elongation lifts stomata of the apophysis above the boundary layer, into the zone of turbulent air, thereby increasing the transpirational pull that draws nutrients across the haustorial foot. The calyptra is interpreted as a gametophytic device to reduce sporophytic demands. The calyptra fits tightly over the intercalary meristem of the sporophytic apex and prevents lateral expansion of the meristem. While intact, the calyptra delays the onset of transpiration. Nutrient transfer across the foot, stomatal number and stomatal aperture are predicted to be particular arenas of conflict between sporophytes and maternal gametophytes, and between maternal and paternal genomes of sporophytes.