chapter 5-9 ecophysiology of development: sporophyte - Bryophyte ...

118 Chapter 5-9: Ecophysiology of Development: SporophyteTradeoffsThe cost of sexual reproduction for the femalecontinues into the cost incurred by the sporophytegeneration. At this point, it seems the cost is even higherthan that of the production of archegonia and eggs. In thecase of Dicranum polysetum (Figure 28), the totalallocation of carbon to sexual reproduction and sporophyteproduction was ~75% (Ehrlén et al. 2000). Whensporophytes were aborted, the top shoots accruedconsiderably more biomass than those shoots wheresporophytes were allowed to complete development,resulting from greater elongation. This large allocation isprobably unusual because this species is one of the fewacrocarpous mosses to produce more than one capsule pergametophyte stem. Like some flowering plants (e.g. Jackin-the-pulpit)that change gender or become sterile in theyear following "fruit" production, the probability ofgametangial production of these D. polysetum plants in thefollowing years was reduced by sporophyte production(Bisang & Ehrlén 2002). Furthermore, annual shootsegments and size of new branches were negativelycorrelated with the development of mature sporophytes.Stark et al. (2000) supported this high cost for sporophytes.They found that the desert moss Syntrichia inermis accruedonly 8% as much mass in aborted sporophytes as it didinthose that matured. Apical sinks of these plants competefor resources needed to produce sporophytes vs producingnew shoots or sexual reproductive structures.spores are produced (Rydgren & Økland 2003). Rydgrenand Økland (2002) point out that there is no evidence of aspore bank or of establishment of new gametophytes fromspores in this species, suggesting that sexual reproductioncomes at a high cost with little benefit. Nevertheless,spores apparently do germinate in new locations followingdisturbance, providing an ecological benefit for the species.The cost or being a reproductive female appears toaffect not only size, but also fitness. In Marchantia inflexa,females are less fit as a result of their narrow window forsuitable timing of the production of gemmae, at least inhigh light (Fuselier & McLetchie 2002). This competitiveenergy drain must necessarily be timed so as not tocompete with energy required for sexual reproduction andsporophyte maturation. Furthermore, selection pressuresthat favor the asexual plants and gemma production maynot coincide with those that favor the sexually maturefemale.Not only does being female reduce the number ofgemmae produced and affect the production of thegametophyte plant, but it can actually be lethal. Followingproduction of capsules, there is a high mortality in the leafyliverwort Lophozia silvicola (Laaka-Lindberg 2000). Innumerous other taxa, having a sporophyte at the apexmeans the end of growth. In the thallose liverwort Blasiapusilla (Figure 29), the parent gametophyte actually diesbefore the sporophyte is mature and the immaturesporophyte overwinters within the dead tissues (Duckett &Renzaglia 1993).Figure 28. Multiple setae per stem on Dicranum polysetum.Photo by Janice Glime.Rydgren and Økland (2002, 2003) found that inHylocomium splendens, the production of sporophyteslikewise reduces the frequency of branching, causes lowermature segment survival and inferior size development tothe next maturity stage, results in fewer immature branchesdeveloping into the first stage of maturity, and fewer plantsproduce new annual segments. Furthermore, the larger,sporophyte-producing branches had significantly lessgrowth than their archegonia-bearing but non-sporophytebearing counterparts. The most expensive stage in thesporophyte development is the late phase when the capsuleexpands, develops its mature color and shape, and theFigure 29. Thallus of Blasia pusilla. Photo by Jan-PeterFrahm.Other tradeoffs are less drastic. In the Pottiales, thereis a negative correlation between life expectancy andprobability of producing sporophytes, but that does notnecessarily imply cause and effect (Hedderson 1995). Onthe other hand, their negative correlation of sporophyteproduction with production of asexual propagules can bethe result of competition for energy reserves.Habitat AdaptationsIt is easy to think of the gametophyte in terms ofadaptations to its habitat, but the sporophyte is oftenneglected in such considerations. As a generationdependent on the gametophyte, it has no choice where todevelop and must therefore cope with the microhabitatprovided for it. Nevertheless, different capsule shapes,sizes, and exposures seem to relate to habitat adaptations.If the sporophyte is adapted for a habitat different from that