Agriculture Reference
In-Depth Information
Chapter 1
Nodulation in a Taxonomic Context
In 2005, Lewis et al. published their comprehensive and beautiful topic 'Legumes of
the World', in which all genera then known are described and at least one species of
each illustrated. In addition, the topic contains a very interesting account of legume
biogeography, which will be the subject of Chapter 2. The general terminology of Lewis
et al. (2005) will be followed here, i.e. the family Leguminosae (or Fabaceae as some
prefer) is divided into three sub-families, each of which is divided into tribes. Table 1.1
summarises these tribes and the numbers of genera and species within them. Since,
with a very few known exceptions, detailed where appropriate, nodulation is a generic
characteristic, for the purposes of the following discussion all species within a genus
are presumed to nodulate, even though the number recorded as nodulated may be far
less than the total (Tables 1.4 to 1.17). At various times when genera have been divided,
it has unwittingly also been on presence or absence of nodulation (Table 1.2). Many of
the more recently described genera in sub-families Mimosoideae and Papilionoideae
have been segregated from others that can nodulate, but often there is no information
on the nodulation status of the new combinations. Whilst, because of their taxonomic
position, many of these are likely to be able to nodulate, in view of the examples given
in Table 1.2, it certainly cannot be taken for granted.
In all three sub-families, there is active research on tribal and generic details. The
authors of the various chapters in Lewis et al. (2005) present the current situation as
they see it, pointing out anomalies without taking a position, because one of their aims
is to stimulate research. In this sense, their topic can be described as covering 'work
in progress'. In some cases nodulation characteristics may add some clarity, and this
will be attempted in the present chapter. A further complication is that the current
ideas of taxonomy, phylogeny and evolution are not entirely congruent with known
major alterations in the chloroplast genome. Doyle (1995) outlined the major changes of
importance for legumes, of which two are relevant here. Chronologically the first is the
inversion of a 50kb section of chloroplast DNA. This is absent from Caesalpinioideae
and Mimosoideae, but present in most Papilionoideae. The second is the loss of a large
duplicated and inverted section of the genome, present in nearly all land plants, and
known as the inverted repeat (IR). Liston (1995) carried out an extensive survey of
this feature of legumes, which is largely confined to the more advanced papilionoid
