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

Abstract

The liverwort Haplomitrium gibbsiae is shown to regularly produce spores released in the form of permanent dyad pairs. Developmental studies indicate that the dyads are produced via a unique half-lobed configuration of the developing sporocyte. Many fossil cryptophytes of Siluro-Devonian age, which are clearly embryophytes based on their morphology, contain permanent spore dyads in their sporangia, but this is the first demonstration of their occurrence in a living plant, a species belonging to Haplomitriopsida, which resolves in a clade that is considered to be sister to all remaining liverworts. Dispersed spore-like dyads are found in the rock record as far back as the mid-Cambrian, but most researchers still regard the first occurrence of isomorphic, tetrahedral tetrads in the mid-Ordovician as the benchmark age for the origin of land plants. Regardless of the geological antiquity of the embryophytes, it appears that H. gibbsiae has retained a non-simultaneous form of sporogenesis that may ultimately be traced to a charophytic origin.

A developmental study of pollen dyads and notes on floral development inScheuchzeria(Alismatales: Scheuchzeriaceae)

“…The rare occurrence of permanent dyads in extant land plants is of special interest because of extensive fossil record of spore or spore-like dyads in mid-Cambrian to early Devonian deposits (Wellman & Gray, 2000;Taylor & Strother, 2008;Renzaglia et al, 2015). Among extant species, the only example of permanent spore dyads is reported for H. gibbsiae, a liverwort belonging to Haplomitriopsida, the class that is considered to be sister to all remaining liverworts (Renzaglia et al, 2015). Spores in dyads of H. gibbsiae are fixed together by simple exine fusion.…”

“…The rare cases of occurrence of permanent dyads in extant land plants are of special interest because of extensive fossil record of spore or spore‐like dyads (along with tetrads and monads) in mid‐Cambrian to early Devonian deposits. The earliest dispersed dyads or dyad packets much predate unequivocal embryophyte fossils (Wellman & Gray, ; Taylor & Strother, ; Renzaglia et al ., ), but some younger specimens indicate occurrence of the dyads in embryophyte sporangia (Edwards, ; Edwards et al ., ). There is no reason to assume that the rare angiosperm dyads are a potentially plesiomorphic feature related to the fossil dyads, but we believe that even comparing analogous situations could be informative.…”

“…R.M.Schust. and in two distantly related groups of angiosperms, Podostemaceae in the eudicot order Malpighiales and Scheuchzeriaceae in the earlydivergent monocot order Alismatales (Walker & Doyle, 1975;Furness & Banks, 2010;Furness, 2012;Lalruatsanga, 2014;Renzaglia et al, 2015). In addition, permanent dyads are reportedly present in some members of the early-divergent angiosperm family Monimiaceae (Takhtajan, 2009) and some Juncaginaceae (Alismatales: Kamelina, 2011), although we failed to find any original descriptions of dyads in these families.…”

Living together and living apart: the sexual lives of bryophytes

“…The formation of bisexual dyads would require mechanisms to ensure that non-sister spores are dispersed together. Persistent dyads have been reported in three species of Haplomitrium [70][71][72][73]. There are two interpretations of the sexuality of these dyads.…”

“…The first interprets the two spores as of opposite sex, ensuring the joint dispersal of the sexes and allowing intra-dyad matings [70,74]. The second interprets the two spores of a dyad as same-sexed products of meiosis II [73]. This question will be resolved empirically, but the first interpretation makes more adaptive sense.…”