Callose is integral to the development of permanent tetrads in the liverwort Sphaerocarpos

Abstract

A striking feature of the liverwort Sphaerocarpos is that pairs of male and female spores remain united in permanent tetrads. To identify the nature of this phenomenon and to test the hypothesis that callose is involved, we examined spore wall development in Sphaerocarpos miche lii, with emphasis on the appearance, location and fate of callose vis-à-vis construction of the sculptoderm. All stages of sporogenesis were examined using differential interference contrast optics, and aniline blue fluorescence to locate callose. For precise localization, specimens were immunogold labeled with anti-callose antibody and observed in the transmission electron microscope. Callose plays a role in Sphaerocarpos spore wall development not described in any other plant, including other liverworts. A massive callose matrix forms outside of the sculptured sporocyte plasmalemma that predicts spore wall ornamentation. Consequently, layers of exine form across adjacent spores uniting them. Spore wall development occurs entirely within the callose and involves the production of six layers of prolamellae that give rise to single or stacked tripartite lamellae (TPL). Between spores, an anastomosing network of exine layers forms in lieu of intersporal septum development. As sporopollenin assembles on TPL, callose progressively disappears from the inside outward leaving layers of sporopollenin impregnated exine, the sculptoderm, overlying a thick fibrillar intine. This developmental mechanism provides a direct pathway from callose deposition to sculptured exine that does not involve the intermediary primexine found in pollen wall development. The resulting tetrad, encased in a single wall, provides a simple model for development of permanent dyads and tetrads in the earliest fossil plants.

Permanent spore dyads are not ‘a thing of the past’: on their occurrence in the liverwortHaplomitrium(Haplomitriopsida)

“…The structural simplicity of the connection between spores of the dyad pair in H. gibbsiae is reminiscent of the exine connections that unite spores in tetrads of Sphaerocarpos Boehm. (Renzaglia et al ., ). In both plants, the spore surface ornamentation extends across spore boundaries, although, because of the regularity of the pattern, it is more prominent in Sphaerocarpos than in H. gibbsiae .…”

“…Intersporal septa are lacking in both, and exine elements in adjacent spores connect directly to each other. In a phylogenetic context, tetrad‐producing Sphaerocarpos represents one of the earliest diverging lineages of complex thalloid liverworts (Forrest et al ., ; Crandall‐Stotler, Stotler & Long, ), and these permanent tetrads may also be viewed as a relict of some of the earliest spore wall developmental processes that arose in land plants (Renzaglia et al ., ).…”

“…Samples were shipped live to Southern Illinois University, Carbondale, IL, USA (SIUC), where elongating and unopened capsules were processed for TEM according to the protocol in Renzaglia et al . (). Capsules were excised from elongated setae and fixed in 2% (v/v) glutaraldehyde in 0.05 m phosphate buffer, pH 7.2, for 1 h at room temperature, followed by three rinses in buffer.…”

“…Although the vast majority of extant embryophytes produce spores that dissociate from their tetrad partners, occasional remnant species produce persistent and even permanent tetrads (Brown et al ., ; Renzaglia, Lopez & Johnson, ). In contrast, there are no published accounts of permanent dyads in extant plants.…”

“…Velatitetras N.D. Burgess) found in Silurian palynofloras (Strother & Traverse, ; Johnson, ; Burgess, ; Wellman, Edwards & Axe, 1998a, b). On the other hand, the rarer permanent free tetrads remain connected independent of the SMC wall (Renzaglia et al ., ); these tetrads are dispersed and germinate as intact units. Thus, they are not enclosed in an envelope and may provide counterparts to even earlier fossil assemblages (Ordovician) that include abundant naked tetrads (Strother et al ., ).…”