BEHNKE, H. - DIETMAR. Zellenlehre, University of Heidelberg, Im Neuenheimer Feld 230, D-69120 Heidelberg, Germany. - Sieve-element plastids and evolution of monocotyledons - with emphasis on Melanthiaceae sensu lato and Aristolochiaceae-Asaroideae, a putative dicotyledon sister group.
Monocotyledons are distinct from dicotyledons by their subtype-P2
sieve-element plastids containing cuneate protein crystals, a
synapomorphic character uniformly present from basal Acorales and
Alismatales through Lilioids to Commelinoids. The dicotyledon genera
Asarum and Saruma (Aristolochiaceae-Asaroideae) are the
only other taxa with cuneate crystals, but their sieve-element
plastids include an additional large polygonal crystal, as typical of
many paleoherbs. New investigations in the Melanthiaceae sensu
lato revealed the same pattern (polygonal plus cuneate crystals)
in the sieve-element plastids of Japonolirion osense
(Nartheciaceae-Petrosavieae / Petrosaviaceae), Harperocallis flava,
Pleea tenuifolia, three Tofieldia species (all:
Nartheciaceae-Tofieldioideae / Tofieldiaceae), and of Narthecium
ossifragum. However, Aletris glabra and Lophiola
americana (Nartheciaceae-Narthecioideae) and all of the15 species
studied and belonging to the Melanthiaceae sensu stricto
contain cuneate crystals only. High resolution TEM pictures reveal a
crystal substructure which in cuneate forms is always densely-packed,
but in polygonal ones is either 'dense' (in Asarum, Saruma, and
Japonolirion) or 'loose' (in Harperocallis, Pleea, and
Tofieldia). In Narthecium 'loose' polygonal crystals
often break up into many small pieces, a process that is also recorded
within the Velloziaceae. Moreover, small 'loose' crystals in addition
to 'dense' cuneate ones are found in a few Lilioids and many taxa of
the Poales (Commelinoids). - These results suggest (1) that form-P2c
sieve-element plastids evolved through the breakup of a single large
polygonal into many cuneate crystals, (2) that this partition was
incomplete, i. e. left over a smaller polygonal crystal which through
steps of (3) loosening and (4) further breakup (5) eventually
dissolved, whereas (6) form-P2cs, -P2cfs and -P2cf subsequently
developed by the addition of starch and/or protein filaments. This
hypothesis is tested against the distribution of the different
subtype-P2 sieve-element plastids and phylogenetic trees derived from
molecular data.
Key words: evolution of monocots, Japonolirion, Melanthiaceae, Nartheciaceae, sieve-element plastids, Tofieldiaceae