Abstract
Should asexual/vegetative offspring be counted in estimates of evolutionary fitness in seed plants? The answer it seems remains debatable among evolutionary ecologists. Arguably, however, the answer is yes when fitness is ascribed strictly to resident genes/alleles, in terms of their survival—through copying and transmission success—over time. This success results only because sporophyte ‘rooted units’ make more ‘rooted units’—not primarily in terms of leaving descendent ‘individuals’, but more fundamentally as carriers of gene/allele copies in perpetuity. The ‘rooted-unit’ (RU) is identified by root and shoot tissue integrated through an essential, and distinctly local, vascular transition. It thus represents a functional and practical circumscription for enumerating individual ‘offspring’ that applies equally to those derived from zygotes, or from clonality. This ‘binary’ nature of fitness estimation in seed plants remains legitimate despite that clonal RU offspring are normally all genotypically identical; the same is true of zygotic/seed-derived RU offspring produced by apomixis or from inbred sexual lines (and somatic mutation sometimes means that clonal offspring are not, in fact, genotypically identical). Neither is this fitness estimation compromised if a young clonal RU offspring, during establishment, temporarily receives resource translocation from connection to a parental RU; the same is true for immature zygotic offspring that receive maternal resources within the seed, via female gametophyte or endosperm/ perisperm tissue. Finally, the above fitness estimate also remains uncompromised even if there is facilitation between mature neighbouring RUs that remain connected with lifetime physiological integration; analogous facilitation also occurs routinely between neighbouring RUs in aclonal species.
Highlights
The relationship between clonality and the definition of an ‘individual’ or an ‘offspring’ in plants was a contentious subject for evolutionary biology in earlier decades
A central question for seed plants is whether evolutionary fitness should be estimated only from counts of sporophyte offspring derived from zygotes, or whether clonal offspring can/should be counted (Fagerstrom 1992, Pan and Price 2002)
Gene/allele copies are transmitted in perpetuity only because parental rooted unit’ (RU) make offspring RUs that can in turn make ‘grand-offspring’ RUs, and so on
Summary
The relationship between clonality and the definition of an ‘individual’ or an ‘offspring’ in plants was a contentious subject for evolutionary biology in earlier decades (e.g. reviewed by Mogie 1992). Both the shoot and root tissue products of a single zygote (genet) are always permanently, and entirely, laterally discontinuous between rooted-units (because 1 genet = 1 RU) (Figure 1a). The RU represents the principal functional vehicle for gene/allele survival (through copying and transmission), and the minimal unit for objectively defining—and for enumeration of— individual sporophyte offspring.
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