2.1. The Consensus Maximum Likelihood (ML) Phylogeny
The basal bifurcation in the consensus ML phylogeny separates Vicia-6 (V. cypria, V. lunata, V. sylvatica, V. ervilia, V. articulata, V. monanthos and V. hirsuta) from all other species with a high degree of certainty (93% bootstrap support). It is this bifurcation and the paraphyly of Vicia that argues for the entire tribe being considered to be the monophyletic genus, Vicia. While this interpretation is well supported, it adds no information regarding the internal relationships of the Fabeae as a whole, which is provided by the recognition of additional genera within the Fabeae.
The reinstatement of the genus,
Ervilla, corresponding to this group of species, branching at node ‘
i’, has the potential to enable the fragmentation of the tribe into more than one genus. These species have not been renamed; for example, the name
Vicia ervilia (L.) Willd. is accepted in The World Checklist of Vascular Plants (WCVP) [
9]. In addition, the remaining
Vicia species are paraphyletic and the other genera are nested within
Vicia.
The creation of another genus,
Ervum, branching at node ‘
e’, has also been proposed [
8]. This genus would comprise
Vicia pubescens, V. tenuissima and
V. tetrasperma (
Vicia1 of
Figure 1). However, this retains a paraphyletic
Vicia, including
Lens. The genus
Ervum is not recognized by WCVP [
9], but
Lens culinaris is no longer accepted and has been replaced by
Vicia lens (L.) Coss. & Germ. Strangely
Lens culinaris subsp.
odemensis (Ladiz.) and
Lens culinaris subsp
. tomentosus (Ladiz.) remain [
10].
The Ervum proposal creates a monophyletic group at node ‘d’ in the consensus ML phylogeny and this group includes Pisum and Vavilovia nested within three Lathyrus lineages. However, this group has only 53% bootstrap support. Pisum and Vavilovia are distinct each with 100% bootstrap support, even though the monospecific genus Vavilovia formosa was formerly Pisum formosum. The combined Pisum plus Vavilovia is monophyletic with 99% bootstrap support, and thus distinct from (or within) Lathyrus.
An important issue to resolve is the status of the nodes labelled ‘
b’, ‘
c’ and ‘
d’ in the ML phylogeny. These do not have strong bootstrap support, raising the possibility that the three groups of
Lathyrus species may be a monophyletic clade, rather that a paraphyletic grade as in this figure. Supposing that
Lathyrus and
Pisum plus
Vavilovia are two monophyletic clades derived at node ‘
d’ (condensing the nodes ‘
b’, ‘
c’, with less than 50% support, and ‘
d’ which musters 53% support), then, in that case, there is no requirement to re-assign any generic status. On the other hand, were there two monophyletic lineages
Lathyrus, as implied by the chronogram of
Figure 1B,then, in that case, the choice would be whether to elevate one subgroup of
Lathyrus to a new genus, or to combine
Lathyrus with
Pisum plus
Vavilovia. Furthermore, it should be noted that the diversity within
Pisum and
Vavilovia is not fully represented in this tree, both with respect to the number of
Pisum subspecies represented [
11,
12,
13] and with respect to the molecular data available. Indeed, whereas
Lathyrus accessions were described both by ITS and plastid DNA sequences, most
Pisum accessions where only characterized by ITS sequences. Fifteen out of 18
Pisum accessions had either one or no chloroplast sequence, which may have significant effects on the robustness of the phylogenetic relationships inferred, especially if sampling of chloroplast data for
Lathyrus is higher.
The majority of
Lathyrus species are represented by
Lathyrus-1 in
Figure 1 and form a monophyletic group, but with 61% bootstrap support; in some trees, some of these species are placed differently; presumably in association with
Lathyrus neurolobus. The third group of
Lathyrus (
L. articulates, L. clymenum and
L. ochrus) is itself distinct and monophyletic with 100% bootstrap support in the ML tree. The arrangement of these three
Lathyrus groups and the
Pisum plus
Vavilovia group is uncertain, with at best 53% bootstrap support for node ‘
d’ and less than 50% support for nodes ‘
b’ and ‘
c’, suggesting that these dichotomies may collapse when adding more data and/or taxa for further phylogenetic inferences (
Figure 1).
2.2. The Chronogram
There are many useful analyses in Schaefer et al. [
8], one of which is a chronogram that attempted to date divergences within the Fabeae. The overall branching structure that relates to the present discussion is given in
Figure 1B. There are several striking features of this structure. The first is that the proposed
Ervilla group is basally branched, but the age estimate of this divergence overlaps slightly with the divergence of
Vicia plus
Lens from
Lathyrus,
Pisum and
Vavilovia. All
Vicia, including the proposed
Ervum, are a monophyletic lineage but distinct from
Lens in this analysis [
8]. This is consistent with retaining
Lens as a separate genus.
In the chronogram, the two minor Lathyrus lineages are distinct from the remaining (and majority of) Lathyrus species, but the age estimate for this divergence overlaps substantially with the estimated age of the divergence of Pisum plus Vavilovia from all Lathyrus. The estimate of the Pisum / Vavilovia divergence time overlaps to some degree with their divergence from the majority of Lathyrus, presumably reflected in the 99% bootstrap support for this clade in the ML tree.