8
Although criticized heavily, the genomic system contributed in the clarifi cation
and understanding of evolutionary relationships within the tribe. Kellogg ( 1989 )
argued that the genomic designations are good indicators of phylogenetic related-
ness , since Löve treated them as discrete character, i.e., as an “all-or-nothing phe-
nomenon” of complete pairing versus nearly complete failure to pair. However,
genome data should never be used solely, but combined with additional data from
other disciplines to overcome differences in genetic regulation of chromosome pair-
ing found among some taxa. In this manner, species groups that share a single
genome and are also morphologically uniform can be considered monophyletic, and
in several cases were found to belong to such groups through later phylogenetic
analyses (e.g., Blattner 2004 ; Petersen et al. 2004 ; Blattner 2009 ). In contrast, het-
erogenomic species (i.e., species that combine two or more different genomes) are
clearly the result of hybridization. Therefore, they are in confl ict with the mono-
phyly criterion of the phylogenetic species concept as more than one parental clade
was involved in their formation. Heterogenomic taxa can, however, be treated as
units of their own in plant groups with a high extent of hybrid formation like
Triticeae. Heterogenomic taxa can be included as reticulations in cladistic analyses,
for review see Kellogg ( 1989 ). Löve and Dewey’s work showed that evolutionary
relationships are not necessarily strictly based on bifurcating ancestral lineages and
hence, cannot be covered by a classifi catory system that allows only for hierarchical
relationships.
1.7 Taxonomic Treatments of the Genus Triticum
There are two different interpretations of the wheat genus “ Triticum ”. Depending on
the taxonomic treatment under use, Triticum either also includes taxa that are other-
wise described under Amblyopyrum and Aegilops (i.e. , Triticum s.l.) or is restricted
to species having the A genome or one of the genome combinations AB , AAB , or
ABD (Table 1.1 ; Barkworth and von Bothmer 2009 ). In the past, Aegilops and
Triticum were rarely subsumed into one genus (Bowden 1959 ; Dvořák and Zhang
1992 ; Yen et al. 2005 ) as proposed by Stebbins ( 1956 ). However , treating them
separately makes Aegilops paraphyletic, as not all descending lineages are included
in the same genus, and Triticum polyphyletic, since Aegilops played a key role in the
formation of tetraploid and hexaploid wheat. Thus, Aegilops tauschii contributed
the D genome to T. aestivum, the B (also referred to as S in the parental species, or
G in other polyploid species) genome stems from Aegilops speltoides (Petersen
et al. 2006 ). Agreement on the congeneric status of Aegilops and Triticum would
solve this nomenclatural problem. But there is a long tradition in keeping both gen-
era separate because of their clear morphological distinctions. Of 153 inspected
fl oristic treatments that were published between 1753 and 1994, approximately
86% considered them as different genera (van Slageren 1994 ). In the following
parts, I adopt the traditional view and treat Triticum as separate from Aegilops (also
following Barkworth and von Bothmer 2009 ).
N. Bernhardt