FIGURE LEGENDS
Figure 1. Pedigree illustration of parental lines
(P1 and P2) and initial filial
generations, where F1 results from crossing the two
parental lines, and F2 results from crossing
F1s. Backcrosses, B1 and
B2, result from crossing F1 with the
parental lines P1 and P2, respectively.
Figure 2. Expectations for the fitness of hybrid
F1 offspring depend on ecological and genetic distances
between parental lineages. To the left, outcrossing of inbred lines
commonly leads to high fitness due to increased heterozygosity (known as
‘genetic rescue’ in conservation genetics). As ecological distance
between parental lines increases, benefits of outcrossing are outweighed
by negative effects of ecological incompatibility. To the right, hybrid
F1 from interspecific crosses commonly show low fitness
due to genetic incompatibilities between highly divergent genomes. In
between these extremes, indicated by the vertical dotted lines, the
relative balance between positive and negative outcomes of both genetic
and ecological divergence generates a rich spectrum of possible fitness
consequences to the hybrid offspring of matings between natural
immigrants and residents arising within metapopulation systems.
Figure 3. Different relative magnitudes of genetic effects can
lead to positive or negative increments on trait or fitness values
across filial generations, as demonstrated by imputing into the
line-cross theory equation (A) arbitrary values for the magnitudes of
genetic effects (C-E, top-right corner). The average trait value between
parental lines (Pmid) indicates the expected trait value for the F1
given purely additive (α1) genetic effects, as in C.
With additive and positive dominance (δ1) effects (as in
D), trait values for all filial generations are larger than Pm,
therefore showing positive mid- and/or best-parent heterosis. Positive
heterosis is also observed for F1 and B1when additive-by-dominance epistasis
(α1δ1) is present in addition to
additive and dominance effects (as in E). In this case, however,
F2 and B2 show negative heterosis due to
the loss of positive epistatic benefits, as indicated by the
coefficients for source and hybridity indexes (B) for these filial
generations (see Box 1 for details). Trait value expectations are
indicated for in environment 1. Note that the y-axes in plots (C-E) are
in the same scale, since the F2 (square) is taken as reference
(μ0).
Figure 4. Non-linear genotype-phenotype maps can lead to
heterosis. A) even if genetic effects are additive, i.e. gene product
(x-axis) for the heterozygote (Aa) equals the mean of the recessive (aa)
and dominant (AA) homozygotes, the phenotypic trait value (Y) for the
heterozygote can deviate from the mean expectation (\(\overline{Y}\)).
This conclusion can be extended to the case of concave adaptive
landscapes where fitness is a non-linear function of the
genotype/phenotype. In the context of local adaptation in
spatially-structured populations (B-C), individuals in populations
P1 (circle) and P2 (square) (presenting
either 100% P1 = 0% P2 alleles or 0%
P1 = 100% P2 alleles) are at the optima
for the different adaptive landscapes corresponding to their respective
local environments (yellow for P1 and blue for
P2). When P1 and P2interbreed (50% P1 alleles), the fitness for the
resulting F1 deviates from the mean fitness of parental
populations (Pmid) in either environment, as a
consequence of the non-linearity of the fitness landscape. In both
examples, populations match “home-vs-away” and “local-vs-foreign”
criteria for patterns of local adaptation, but fitness decreases more
abruptly in the adaptive landscapes of B than of C, representing more
contrasting selective pressures between environments as the optima are
further apart. Consequently, while scenario C leads to positive
heterosis, hybrid offspring in B show outbreeding depression.
Figure 5 . Representation of literature review work pipeline.
The flow diagram (left) depicts each stage of the process with the total
number of studies advancing to the next stage displayed over the
respective box. The Venn diagrams (right) show the numbers of papers
found per keywords searched (colour codes) for two different stages of
the process: total papers triaged (upper; N=12,862), and studies
selected as containing fitness or fitness-related estimates of offspring
from intra- and inter-population crosses (lower; N=111). Although a
large total number of studies was found, the Venn diagrams indicate
little overlap between results produced by keywords chosen to find
ecological studies versus keywords representing more commonly used terms
within traditional heterosis literature. See text and table 1 for
details.
Figure 6. Proportion of studies on major plant and animal
groups for which estimates of fitness or fitness-related traits for both
within and between population crosses were found in the literature
review. In the outer pie, percentages are shown for groups that appeared
in 5% (i.e. N > 5) or more studies out of the total number
of studies using animals (N=32) or plants (N=79).
Figure 7. The literature review indicates a paucity of
estimates of fitness consequences of interbreeding between natives and
immigrants of populations connected by dispersal in natural
environmental conditions. Studies found included population crosses
across different levels of connectivity, with some (total number of
studies presented above the bars) containing crosses between populations
connected (“Connected” bars) or not connected (“Not connected”) by
natural dispersal, some including crosses at both connectivity levels
(“Both”), and some for which connectivity was not possible to be
categorized (“Unclear”). For each connectivity category, left bar
indicates approach used to obtain crosses between populations.
“Introduction” indicates the translocation of individuals from a
different population, or [re-]introduction of individuals from
different populations into an uninhabited site. Right bars indicate the
type of environment in which at least one fitness component or
fitness-related trait was estimated for filial generations obtained via
experimental crosses: “parental” indicates at least one of the
parental environments, “semi-natural” indicates mesocosms, common
gardens, or environments not occupied by the parental populations, and
“artificial” indicates laboratory or greenhouse. Percentages shown are
in reference to the total number of studies within each connectivity
category using experimental crosses (left) or using experimental crosses
and estimating fitness exclusively in artificial or semi-natural
environments (right).