Agriculture Reference
In-Depth Information
Chapter 3
Evolution of Nodulation
The first part of this chapter will concentrate on the plant side of the symbiosis, con-
sidering in turn, when, where, how and why many legumes adopted a symbiotic life
style with soil bacteria. This is followed by a short section on the bacterial partner and
ends with a brief consideration of the role of 'model legumes' in unravelling some of
the major questions of nodule formation.
3.1 When did nodulation first occur?
Nodulation is a phenomenon restricted to the Eurosid 1 clade of angiosperms
(Fig. 3.1). Althoughnot included in this analysis,
Parasponia
, the one non-legume known
to nodulate with rhizobia, is in the Rosales. When the ancestors of this clade first ap-
peared is not entirely clear, but was likely to have been at least 100 million years ago.
For a discussion of this and when plants nodulating with
Frankia
(actinorhizal plants,
shown in bold in Fig. 3.1) may have evolved, see Swensen and Benson (2007). Why the
occurrence of actinorhizal symbioses is so scattered is unclear, as is why only one of
the two main branches of the Fabales led to nodulation. Even within this nodulating
branch (the Leguminosae) things are far from simple.
A great deal of research has been conducted over the last 15 or so years, matching
molecular, morphological and anatomical data to produce possible legume phyloge-
nies. Based on these data and taking into account well-established legume fossil data
to root trees in a likely time frame, Lavin et al. (2005) published a scenario for the
evolution of the major tribes of legumes. Evidence suggests that the first legumes ap-
peared about 59 Ma, in the early Tertiary period, rather than in the late Cretaceous
period as once thought. Within the next 1 to 2 million years, elements of caesalpinioid
and papilionoid legumes evolved, with mimosoids branching from the caesalpinioids
rather later (about 42 Ma). The chronology of the major events involving nodulation is
summarised in Table 3.1.
The total number of species analysed by Lavin et al. (2005) is impressively large,
but does not include some important nodulating genera. In Caesalpinioideae, only
