General G ST and θ inflation due to biased intra-population sampling, and its consequences for the conservation of the Canarian Flora

Abstract

While knowledge of the degree of inter-population genetic differentiation underlies the understanding of microevolutionary processes in any organism, its calculation through G ST , F ST , or h (which, unlike the previous two, was designed to correct for unequal and small sample sizes) is often based in severely restricted intra-population samples, which are nonetheless tacitly assumed adequate to their accurate estimation. Empirical assessment of the influence of the number and intra-population distribution of samples on the values of G ST and h for several Canarian endemic plants compellingly suggests that (1) contrary to expectations based on simulated datasets, h does not account for empirical sampling bias better than G ST ; (2) sample sizes being equal, collections scattered across each population's occupancy area entail significantly lower over-estimates of G ST and h than if they only consider one of the population extremes, especially in narrow allogamous taxa with small populations; (3) in small samples, a scattered sampling strategy is significantly less sensitive to G ST inflation than sampling in one of the population extremes; and (4) a software-related component of bias should be considered when pooling values of G ST from different studies to calculate averages. Thus, unlike the sampling methods used for many plant endemics from the Canaries and other regions, collections for a reliable estimation of inter-population differentiation using molecular markers should encompass the whole occupancy area of each population, and include a higher proportion of individuals respect to the total size in narrow endemics than in widespread congeners. Critically, the high average allozyme inter-population differentiation reported for the Canarian endemic Flora is possibly an over-estimate, and could be explained predominantly by the generally biased intrapopulation sampling associated with G ST estimates, rather than by specific factors of insularity that restrict gene flow radically, as it has been hitherto assumed.

Palaeo‐islands as refugia and sources of genetic diversity within volcanic archipelagos: the case of the widespread endemic Canarina canariensis (Campanulaceae)

“…Where possible, we collected a minimum of 10 samples per population. To reduce inflation in gene descriptors due to biased sampling (Caujapé‐Castells ), samples were collected from individuals scattered across the whole occupancy area of each population. DNA from 160 individuals and preserved in silica gel was extracted using the DNeasy Plant Mini Kit (QIAGEN Inc., Valencia, CA, USA), from 20 to 25 mg of silica‐gel‐dried leaves obtained from the fresh plant tissue collected from the field expeditions.…”

Conservation of oceanic island floras: Present and future global challenges

“…Consequently, both studies called for more intensive conservation measures on the grounds that alterations of genetic connectivity could cause genetic homogenization, severe reductions in genetic diversity among populations, or hybridization (Francisco-Ortega et al, 2000). Nonetheless, estimates of high differentiation among natural populations in many published studies could be a spurious consequence of biased intra-population sampling, rather than a true representation of the biological characteristics of these insular floras (Caujapé -Castells, 2009a).Although the number of papers describing genetic variation in island populations has mushroomed in the last 10 years, very few of them include mainland congeners in comparisons. Hence, we lack robust measurements of the genetic depauperation of island taxa.…”

Conservation implications of high genetic variation in two closely related and highly threatened species ofCrambe(Brassicaceae) endemic to the island of Gran Canaria:C. tamadabensisandC. pritzelii

“…The interpopulation apportionment of genetic variability, as inferred from Nei's G ST values (0.067 for C. tamadabensis and 0.126 for C. pritzelii ) and AMOVA results (Table ), indicate a high genetic cohesion within each species, in sharp contrast with the high fragmentation levels estimated for the Canarian flora ( G ST = 0.280; Francisco‐Ortega et al ., ). Caujapé‐Castells () and Pérez de Paz & Caujapé‐Castells () indicated that many of the average values of G ST reported by Francisco‐Ortega et al . () might be overestimates resulting from a restricted sampling in terms of the number and distribution of individuals.…”

“…are substantially higher than the averages for the Canarian endemics included in the review of Pérez de Paz & Caujapé‐Castells (), who emphasized the high genetic variability of the Canarian flora. Considering that many species included in that review were not sampled intensively, it is probable that the average genetic diversity in the Canarian flora is still much higher than reported [see Caujapé‐Castells () and Pérez de Paz & Caujapé‐Castells ()].…”

“…Furthermore, the general hypothesis that interpopulation genetic variation is much higher in island endemics is at odds with the finding of low interpopulation genetic diversity in species featuring high genetic diversity levels. An investigation of Canarian endemics (Caujapé‐Castells, ) has strongly suggested that the detection of high levels of genetic fragmentation, as measured by G ST (Nei, ) or F ST (Wright, ), may be strongly influenced by a spatially non‐representative intrapopulation sampling.…”