Toshihiro Yamada

Dissection of two major components of the post‐zygotic hybridization barrier in rice endosperm

A post-zygotic hybridization barrier is often observed in the endosperm of seeds produced by interspecific or interploidy crosses. In Arabidopsis thaliana, for example, hybrid endosperm from both types of cross shows altered timing of cellularization and an altered rate of nuclear divisions. Therefore, it has been proposed that interspecific and interploidy crosses share common molecular mechanisms for establishment of an effective species barrier. However, these two types of hybridization barrier may be initiated by different intrinsic cues: the interspecific cross barrier arises after hybridization of genomes with differences in DNA sequences, while the interploidy cross barrier arises after hybridization of genomes with the same DNA sequences but differences in ploidy levels. In this study, we performed interploidy crosses to identify components of the post-hybridization barrier in the endosperm of rice. We performed an intra-cultivar cross of autotetraploid (4n) × diploid (2n) rice, and found precocious cellularization and a decreased rate of nuclear division in the syncytial endosperm. By contrast, seeds from the reciprocal cross showed delayed cellularization and an increased rate of nuclear division. This differential effect on nuclear division rates contrasts with the outcome of rice interspecific crosses, which were previously shown to have altered timing of cellularization without any change in nuclear division rates. Thus, we propose that the post-zygotic hybridization barrier in rice endosperm has two separable components, namely control of the timing of cellularization and control of the nuclear division rates in the syncytial stage of endosperm development.

The plant fossils from the Kaizara Formation (Callovian, Jurassic) of the Tetori Group in the Izumi district, Fukui Prefecture, Central Japan

ABSTRACT The Middle Jurassic (Bathonian to Callovian) Kaizara flora is proposed herein for the plant fossil assemblage in the Kaizara Formation, Kuzuryu Subgroup, Tetori Group. In addition to Otozamites crassipinnatus sp. nov., twelve species are reported, including species of Equisetales, uncertain order of pteridosperms, Cycadeoideales, Cycadales, and Coniferales. Some species have very thick lamina, implying that the climate might include dry periods. All species found in the Kaizara flora are new to the Tetori Group. Therefore, the Tetori-type flora is restricted to post-Callovian stages in the Tetori Group.

Oldest record of Trimeniaceae from the Early Cretaceous of northern Japan

BackgroundMolecular phylogenetic analyses have identified Trimeniaceae, a monotypic family distributed only in Oceania, as among the earliest diverging families of extant angiosperms. Therefore, the fossils of this family are helpful to understand the earliest flowering plants. Paleobotanical information is also important to track the historical and geographical pathways to endemism of the Trimeniaceae. However, fossils of the family were previously unknown from the Early Cretaceous, the time when the angiosperm radiated. In this study, we report a seed from the late Albian (ca. 100 million years ago) of Japan representing the oldest known occurrence of Trimeniaceae and discuss the character evolution and biogeography of this family.ResultsA structurally preserved seed was collected from the early Late Albian Hikagenosawa Formation of the Yezo Group, which was deposited in palaeolatitudes of 35 to 40°N. The seed has a multilayered stony exotesta with alveolate surface, parenchymatous mesotesta, and operculate inner integument, which are characteristic to extant trimeniaceous seeds. However, the seed differs from extant seeds, i.e., in its well-developed endosperm and absence of antiraphal vascular bundle. Thus, the seed would be a new genus and species of Trimeniaceae.ConclusionThe fossil seed indicates that seed coat characters were conserved for 100 million years or more in Trimeniaceae. It also suggests that the antiraphal vascular bundle and perispermy originated secondarily in Trimeniaceae as previously inferred from the phylogeny and character distribution in the extant basalmost angiosperms. The fossil seed provides the first evidence that Trimeniaceae was distributed in a midlatitude location of the Northern Hemisphere during the Early Cretaceous, when angiosperms radiated extensively, supporting a hypothesis that the extant austral distribution is relict.

Expression patterns of class I KNOX and YABBY genes in Ruscus aculeatus (Asparagaceae) with implications for phylloclade homology

STM (RaSTM) and YAB2 (RaYAB2) homologues were isolated from Ruscus aculeatus (Asparagaceae, monocots), and their expressions were analyzed by real-time polymerase chain reaction (PCR) to assess hypotheses on the evolutionary origin of the phylloclade in the Asparagaceae. In young shoot buds, RaSTM is expressed in the shoot apex, while RaYAB2 is expressed in the scale leaf subtending the shoot bud. This expression pattern is shared by other angiosperms, suggesting that the expression patterns of RaSTM and RaYAB2 are useful as molecular markers to identify the shoot and leaf, respectively. RaSTM and RaYAB2 are expressed concomitantly in phylloclade primordia. These results suggest that the phylloclade is not homologous to either the shoot or leaf, but that it has a double organ identity.

AINTEGUMENTA homolog expression in Gnetum (gymnosperms) and implications for the evolution of ovulate axes in seed plants

SUMMARY The expression of GpANTL1, a homolog of AINTEGUMENTA (ANT) found in the gymnosperm Gnetum parvifolium, was analyzed by RT‐PCR and in situ hybridization. GpANTL1 was expressed in the leaf primordia, root tips, and young ovules. In the ovulate axis, expression was detected as four distinct rings around the outer, middle, and inner envelope primordia as well as around the nucellar tip. This pattern of expression is similar to that of ANT in Arabidopsis thaliana. A comparison of the expression of GpANTL1 with that of PtANTL1 in the conifer Pinus thunbergii suggests that the integrated expression of PtANTL1 may have been caused by congenital fusion of the integument, ovuliferous scale, and bract.

CORONA, PHABULOSA and PHAVOLUTA collaborate with BELL1 to confine WUSCHEL expression to the nucellus in Arabidopsis ovules.

Angiosperm ovules consist of three proximal-distal domains – the nucellus, chalaza and funiculus – demarcated by developmental fate and specific gene expression. Mutation in three paralogous class III homeodomain leucine zipper (HD-ZIPIII) genes leads to aberrations in ovule integument development. Expression of WUSCHEL (WUS) is normally confined to the nucellar domain, but in this triple mutant expression expands into the chalaza. MicroRNA-induced suppression of this expansion partially suppresses the effects of the HD-ZIPIII mutations on ovule development, implicating ectopic WUS expression as a component of the mutant phenotype. bell1 (bel1) mutants produce aberrant structures in place of the integuments and WUS is ectopically expressed in these structures. Combination of bel1 with the HD-ZIPIII triple mutant leads to a striking phenotype in which ectopic ovules emerge from nodes of ectopic WUS expression along the funiculi of the primary ovules. The synergistic phenotype indicates that BEL1 and the HD-ZIPIII genes act in at least partial independence in confining WUS expression to the nucellus and maintaining ovule morphology. The branching ovules of the mutant resemble those of some fossil gymnosperms, implicating BEL1 and HD-ZIPIII genes as players in the evolution of the unbranched ovule form in extant angiosperms. Summary: Transgenic analysis identifies transcription factors necessary for boundary demarcation between the nucellus and chalaza of the Arabidopsis ovule, and for patterning of WUS expression.

Comparative anatomy of embryogenesis in three species of Podostemaceae and evolution of the loss of embryonic shoot and root meristems.

SUMMARY During embryogenesis in angiosperms, the embryonic shoot and root meristems are created at opposite poles of the embryo, establishing a vertical body plan. However, the aquatic eudicot family Podostemaceae exhibits an unusual horizontal body plan, which is attributed to the loss of embryonic shoot and root meristems. To infer the embryogenetic changes responsible for the loss of these meristems, we examined the embryogenesis of three podostemads with different meristem characters, that is, Terniopsis brevis with distinct shoot and root meristems, Zeylanidium lichenoides with reduced shoot and no root meristems, and Hydrobryum japonicum with no shoot and no root meristems. In T. brevis, as in other eudicots, the putative organizing center (OC) and L1 layer (=the epidermal cell layer) arose to generate a distinct shoot meristem initial, and the hypophysis formed the putative quiescent center (QC) of a root meristem. Z. lichenoides had a morphologically unrecognizable shoot meristem, because a distinct L1 layer did not develop, whereas the putative OC precursor arose normally. In H. japonicum, the vertical divisions of the apical cells of eight‐cell embryo prevented putative OC initiation. In Z. lichenoides and H. japonicum, the putative QC failed to initiate because the hypophysis repeated longitudinal divisions during early embryogenesis. Based on their phylogenetic relationships, we infer that the conventional embryonic shoot meristem was lost in Podostemaceae via two steps, that is, the loss of a distinct L1 layer and the loss of the OC, whereas the loss of the embryonic root meristem occurred once by misspecification of the hypophysis.

Root apical meristem diversity in extant lycophytes and implications for root origins

Root apical meristem (RAM) organization in lycophytes could be a key to understanding the early evolution of roots, but this topic has been insufficiently explored. We examined the RAM organization of lycophytes in terms of cell division activities and anatomies, and compared RAMs among vascular plants. RAMs of 13 species of lycophytes were semi-thin-sectioned and observed under a light microscope. Furthermore, the frequency of cell division in the RAM of species was analyzed using thymidine analogs. RAMs of lycophytes exhibited four organization types: type I (Lycopodium and Diphasiastrum), II (Huperzia and Lycopodiella), III (Isoetes) and RAM with apical cell (Selaginella). The type I RAM found in Lycopodium had a region with a very low cell division frequency, reminiscent of the quiescent center (QC) in angiosperm roots. This is the first clear indication that a QC-like region is present in nonseed plants. At least four types of RAM are present in extant lycophytes, suggesting that RAM organization is more diverse than expected. Our results support the paleobotanical hypothesis that roots evolved several times in lycophytes, as well as in euphyllophytes.

Fossil records of subsection Pinus (genus Pinus, Pinaceae) from the Cenozoic in Japan

Extant pines of subsection Pinus (section Pinus, genus Pinus, Pinaceae) are predominantly distributed in Eastern Asia. However, the extent of diversification in the section has yet to be fully clarified. We reviewed fossil records of subsection Pinus from Japan and collected permineralized materials, in which anatomical details are preserved for better understanding of the diversification. Our results suggest that this subsection appeared in Japan no earlier than the Middle Eocene, with extant species (i.e., Pinus densiflora and Pinus thunbergii) appearing around the beginning of the Pleistocene. Pinus fujiii (Early Miocene to Early Pleistocene) is inferred to have a close affinity to P. thunbergii based on the medial arrangement of its leaf resin canals. Additionally, P. fujiii has a similar cone morphology to those of extant species living in China, bridging the morphological gap between P. thunbergii and Chinese relatives of P. thunbergii as inferred by molecular phylogenetic analyses. Our results also suggest that taxonomic revisions of Pinus miocenica and Pinus oligolepis are required among the Japanese fossil species reported to date.

Platanoid leaves from Cenomanian to Turonian Mikasa Formation, northern Japan and their mode of occurrence

ABSTRACT Ettingshausenia cuneifolia is reported from the Cenomanian to Turonian Mikasa Formation of central Hokkaido. These leaves occur exclusively in carbonaceous mudstone deposited in either lagoon or marsh. Based on their mode of occurrence, these remnants may have been trapped in the sediments without long prior transport (parautochthonous).