Geng, van Klinken, Sosa, Li, Chen, & Xu (2016) — The Relative Importance of Genetic Diversity and Phenotypic Plasticity in Determining Invasion Success of a Clonal Weed in the USA and China

Bibliographic Reference

Geng, Y., van Klinken, R. D., Sosa, A., Li, B., Chen, J., & Xu, C.-Y. (2016). The relative importance of genetic diversity and phenotypic plasticity in determining invasion success of a clonal weed in the USA and China. Frontiers in Plant Science, 7, Article 213. https://doi.org/10.3389/fpls.2016.00213

Core Argument

Invasive clonal plants present a paradox: they often exhibit low genetic diversity due to founder effects and predominantly clonal (asexual) propagation, yet they successfully colonize diverse and heterogeneous environments across broad geographic ranges. Phenotypic plasticity — the ability of one genotype to express different phenotypes in different environments — has been proposed as an adaptive strategy that may compensate for low genetic diversity, but the relative importance of these two mechanisms for invasion success remained largely untested.

The study uses the globally important clonal weed Alternanthera philoxeroides (alligator weed) as a model system, comparing populations from the native range (Argentina) with two introduced ranges that have different invasion histories (the USA and China). The central hypothesis is that if genetic diversity drives invasion success, then introduced populations with lower genetic diversity should show restricted niche breadth. Conversely, if phenotypic plasticity is the primary driver, then even genetically depauperate populations should achieve full niche occupancy through plastic responses to environmental variation.

Methods

The study employed three complementary approaches:

Molecular marker analysis: Genetic diversity was assessed using eight Inter Simple Sequence Repeat (ISSR) markers on 179 individuals collected from Argentina (7 sites, n=21), the USA (9 sites, n=32), and China (9 sites, n=126). Bands were scored as present/absent, and three genetic variables were calculated: percentage of polymorphic loci (P), Nei’s genic diversity index (He), and Shannon diversity index (I). A resampling procedure controlled for uneven sample sizes. A neighbor-joining tree was constructed using PAUP 4.0 with 1000 bootstrap replicates.

Common garden experiment: Twenty-five clones (one per sampling site: 7 from Argentina, 9 from the USA, 9 from China) were grown under two water availability treatments — aquatic (1 m deep ponds) and terrestrial (raised garden beds) — at Fudan University, Shanghai. Eight morphological and physiological traits were measured: leaf length, stem diameter, stem pith cavity diameter, internode length, specific leaf area (SLA), relative chlorophyll content (SPAD), root/shoot ratio, and storage root/fine root ratio. Phenotypic plasticity was analyzed using two-way nested ANOVA, plasticity indices (Ip = (Max-Min)/Mean), and principal component analysis (PCA). Mantel tests assessed correlation between genetic marker distance and quantitative trait dissimilarity.

Bioclimatic modeling: A CLIMEX model was fitted against native-range (Argentina) distribution data and used to determine whether the full potential bioclimatic distribution had been invaded in China and the USA.

Key Findings

• “Chinese clones had very low genetic diversity in terms of both marker diversity and quantitative variation when compared with those from the USA and Argentina, probably reflecting different introduction histories.” (Abstract, lines 103-104 of extracted text)

• “94% (119) of the Chinese samples were identical (referred as to ‘C-Dominant’)” and “the eight Chinese genotypes clustered together as a single well-supported clade in the neighbor-joining tree.” (Results, lines 554-557) In contrast, each plant from Argentina and the USA was characterized by a unique multi-locus genotype, and USA clones were intermingled with Argentine clones, suggesting multiple introductions.

• “In contrast, similar patterns of phenotypic plasticity were found for clones from all three regions.” (Abstract, lines 104-105) The two-way ANOVA revealed “significant effects of treatment on all the traits, indicating significant phenotypic plasticity across all regions” (Results, lines 622-626), but no significant treatment-by-region interaction for most traits, indicating comparable plasticity levels.

• “Bioclimatic modeling suggested that the full potential distribution of the species in the introduced ranges were invaded in China and the USA.” (Results, lines 660-662) Despite vastly different levels of genetic diversity, both introduced ranges occupied their predicted climatic niche.

• “Phenotypic plasticity, not genetic diversity, was therefore critical in allowing A. philoxeroides to invade diverse habitats across broad geographic areas.” (Abstract, lines 107-109) The PCA showed that “most (67.61%) of the phenotypic variation within the common garden experiment was a plastic response to habitat treatment.” (Results, lines 655-656)

• “We detected no significant correlation between marker distance and dissimilarity of phenotypic plasticity across terrestrial and aquatic habitats (r = 0.15, p = 0.29)” (Results, lines 639-641), suggesting the plastic response norm is an inherent (species-level) acclimation rather than a trait under local genetic control.

Concepts Introduced or Used

• Phenotypic plasticity — The capacity of a single genotype to produce different phenotypes in response to environmental conditions. The paper positions this as the primary adaptive mechanism allowing clonal invaders to occupy diverse habitats despite low genetic diversity.
• Genetic diversity — Variation at the genetic level, measured both by neutral molecular markers (ISSR) and quantitative trait variation under common garden conditions. The paper contrasts marker diversity with quantitative variation.
• Clonal propagation — Asexual reproduction producing genetically identical offspring (ramets). A. philoxeroides relies predominantly on clonal propagation in introduced ranges, producing viable seeds rarely or not at all.
• Invasion success — Operationalized as the extent to which a species occupies its potential bioclimatic distribution in introduced ranges, assessed by CLIMEX modeling.
• Common garden experiment — An experimental design where individuals from different populations are grown under identical conditions to distinguish genetic from environmental sources of phenotypic variation.
• Founder effects — Loss of genetic diversity when a new population is established by a small number of individuals from a larger source population. The paper invokes founder effects to explain the extreme genetic uniformity of Chinese populations.
• Multiple introductions — Repeated introductions from different source populations, which can maintain or increase genetic diversity in introduced ranges. Used to explain the high genetic diversity of USA populations.
• Reaction norm — The pattern of phenotypic expression of a genotype across a range of environments, visualized as a line plotting phenotype against environment.
• Plasticity index (Ip) — A quantitative measure of phenotypic plasticity calculated as (Max - Min)/Mean for a given trait across two treatments.
• Bioclimatic modeling / CLIMEX — A modeling approach that uses climate variables (temperature, soil moisture) to predict the potential geographic distribution of a species based on its native-range climate envelope.

Entities Referenced

• Alternanthera philoxeroides (Mart.) Griseb. — Alligator weed, a perennial stoloniferous herb native to South America, invasive in over 30 countries. The study species.
• ISSR (Inter Simple Sequence Repeat) markers — Eight primers from the University of British Columbia primer set nine (UBC no. 811, 813, 823, 835, 840, 841, 880, and 887).
• Geographic regions — Argentina (native range, seven sites along the Parana, Uruguay, San Borombon, and Salado rivers); USA (introduced, nine sites in southern coastal plains from Virginia to Texas and California); China (introduced, nine sites across most provinces south of the Yellow River).
• CLIMEX Version 4.0 — Bioclimatic modeling software used with the world 10-minute climate dataset from CliMond.
• Popgene 1.32 — Software for genetic diversity analysis (P, He, I).
• PAUP 4.0 — Phylogenetic analysis software for neighbor-joining tree construction.

Limitations (as stated by authors)

• “The regional-level genetic diversity in USA and Argentina may be underestimated due to smaller sample size, which means the overall pattern of genetic diversity between China and the other two regions may be even more prominent.” (Discussion, lines 790-793)
• “Although, it is not easy to rigorously confirm the adaptive significance of phenotypic plasticity in non-model species” (Discussion, lines 927-929), the authors address this by citing prior evidence that the plastic root/shoot ratio is “true plasticity” rather than “ontogenetic drift.”
• The CLIMEX model “greatly overestimated the native-range distribution of A. philoxeriodes, suggesting that other factors such as topography and competition are important in limiting the distribution of A. philoxeriodes in the native range.” (Discussion, lines 1049-1053)
• The plasticity index for Chinese clones “might not be independent to each other” since all Chinese clones in the common garden experiment were the same multi-locus genotype (C-Dominant). (Methods, lines 454-457) The authors addressed this with supplementary nested ANOVA and t-test analyses that produced similar results.
• The bioclimatic model did not incorporate “other factors such as topography and competition” (Discussion, lines 1052-1053) that are likely important in limiting the native-range distribution.
• The experimental design included only two water treatments (terrestrial and aquatic), which may not capture the full range of environmental heterogeneity experienced by natural populations.
• The authors note that “the role of genetic diversity in invasion success might be variable for plant species with different reproductive modes” (Discussion, lines 903-905), limiting generalization from a single clonal species.

Relevance to Clonal Evolution

This paper is the first ecological source in the wiki corpus, and its relevance to clonal evolution in cancer is cross-domain rather than direct. The paper does not mention cancer. Its significance lies in providing empirical evidence for a dynamic formally analogous to claims made on the dual-regime-evolution page:

1. Genetic homogeneity does not preclude adaptive success. Chinese A. philoxeroides populations are nearly genetically uniform (94% of sampled individuals share one multi-locus genotype), yet they have invaded the full predicted bioclimatic range in China. In cancer terms, this maps onto the observation that monoclonal or oligoclonal tumor architectures can nonetheless exhibit phenotypic heterogeneity sufficient for adaptation — as when a single clone produces diverse drug-tolerant persister states through epigenetic mechanisms without requiring new genetic variation.

2. Phenotypic plasticity as a compensatory mechanism when genetic diversity is low. The paper’s central finding — that plasticity, not diversity, drives invasion success — directly supports the dual-regime model’s claim that the epigenetic (non-Darwinian) regime can drive adaptation independently of genetic (Darwinian) change. The parallel is structural: in both systems, a population experiences a bottleneck (founder event in invasion; treatment bottleneck in cancer) that collapses genetic diversity, yet adaptation proceeds through plastic phenotypic adjustment.

3. Cross-domain functorial mapping. The relationship between the ecological invasion system and the cancer system can be formalized using the category-theoretic framework of buehler2011-reoccurring-patterns. The mapping preserves structure across domains:

Ecology (Geng et al. 2016)	Cancer evolution
Clonal plant population	Tumor cell population
Founder effect (single introduction)	Treatment bottleneck / monoclonal architecture
Low ISSR marker diversity	Low mutational heterogeneity
Phenotypic plasticity (morphological traits)	Epigenetic plasticity (chromatin remodeling, gene expression)
Water availability (terrestrial vs. aquatic)	Therapeutic pressure (drug vs. no drug)
Bioclimatic niche occupancy	Metastatic colonization of distant sites

4. Distinct introduction histories produce distinct outcomes. The finding that USA populations had high genetic diversity (multiple introductions) while Chinese populations had extremely low diversity (single introduction) — yet both achieved full potential distribution — mirrors the observation in cancer that tumors with different evolutionary histories (e.g., branched vs. linear evolution) can reach similar adaptive endpoints. The paper highlights that “the pattern of ‘lower genetic diversity’ in one introduced range (i.e., China) was not found in another introduced range (i.e., the USA), reflecting the heterogeneous nature of biological invasions even for the same invader” (Discussion, lines 1072-1074), which maps onto the finding in compression-progress-evolution that evolutionary trajectories are path-dependent but converge on functionally similar outcomes.

5. Implications for parsimony in evolutionary explanations. The paper demonstrates that a relatively simple mechanism (inherent species-level phenotypic plasticity) explains invasion success without requiring local adaptation or post-invasion genetic evolution. This parallels the dual-regime claim that the non-Darwinian regime may be the more parsimonious explanation for observed tumor adaptation in many clinical contexts — that invoking selection on genetic variation is unnecessary when epigenetic plasticity suffices.

Caveat: This is a single-species study in an ecological context. The mapping to cancer is analogical, not homologous. Direct experimental tests of the plasticity-compensates-for-diversity hypothesis in cancer systems (e.g., comparing isogenic cell lines under drug pressure) would be required to validate the cross-domain inference. Nonetheless, the paper provides strong independent-domain evidence that the dual-regime framework’s core claim — that adaptive success can be achieved through non-genetic mechanisms in genetically homogeneous populations — is empirically plausible in at least one biological system.

Revision history

• 2026-07-05 — Source summary created from Geng et al. (2016). First ecological source in the wiki corpus. Provides cross-domain empirical evidence for the dual-regime model: phenotypic plasticity compensates for low genetic diversity in successful invaders. (geng2016-genetic-diversity-phenotypic-plasticity)