Introgression in Central American Phytolacca (Phytolaccaceae)

Davis, Jerrold I.

doi:10.2307/2443611

Cited by 3 publications

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“…The timing ofbract growth and nectary development may be shifted in the hybrids as compared to parental species, with nectary position set in the hybrids relatively early in bract development. Similar arguments have been proposed to explain extreme conditions of the inflorescence in hybrid Phytolacca (Davis, 1985), of the shell in hybrid snails (Cerion. Woodruff and Gould, 1987), and of meristic counts in salmonid fish hybrids (Leary et al, 1985).…”

Section: Genetic and Evolutionary Implications Of Character Expressiosupporting

confidence: 69%

“…-Morphological intermediacy is often used as evidence in support of hypotheses on hybridity of "suspect" organisms (e.g., Davis, 1985;Tortosa, 1988;Wagner, 1983), although Neff and Smith (1979), among others, have pointed to the potential circularity of this criterion. Wagner (1983) has noted that the intermediacy of hybrids may make it difficult to distinguish between hybrids and "normal" taxa that are intermediate in character states between the extremes present in the group.…”

Section: Implications Ofaphelandra Hybrids For Phylogenetic Analysismentioning

confidence: 99%

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Hybrids and Phylogenetic Systematics I. Patterns of Character Expression in Hybrids and Their Implications for Cladistic Analysis

McDade

1990

Evolution

115

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Abstract. -Interspecific hybridization is considered common among plants, but the methods of cladistic systematics produce only divergently branching phylogenetic hypotheses and thus cannot give the correct phylogeny ifan analysis includes hybrids. Empirical studies ofthe impact ofknown hybrids on phylogenetic analysis are lacking, and are necessary to begin to understand the problems that we face if hybrids are often included in cladistic analysis. Examination of the implications of hybrids for cladistics must begin with patterns of character expression in hybrids. This study includes 17 hybrids and their nine parental taxa that are Central American species ofAphelandra (Acanthaceae), analyzed using a set of50 morphological characters. The hybrids are overwhelmingly intermediate as quantitatively scored for phylogenetic analysis. They express maternal and paternal, and primitive and derived characters in equal frequencies, showing no evidence of predominant inheritance of derived character states as has been assumed by most cladists who have considered hybrids theoretically. Because oftheir known genetic constitution, hybrids were useful in homology assessment and ordering character states. The parental character set was generally robust, but some changes were made to reflect the special evidence offered by the hybrids. These hybrids suggest that the inclusion of hybrids in phylogenetic analysis will not lead to unresolved cladograms with rampant homoplasy, as has been predicted by other authors. Instead, the patterns of character inheritance in these hybrids lead to the prediction that a hybrid will be placed by phylogenetic analysis as a basal lineage to the clade that includes its most derived parent, with relatively little effect on homoplasy. These predictions will be evaluated by incorporation of the hybrids in phylogenetic analyses, to be reported in a subsequent paper.

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Section: Genetic and Evolutionary Implications Of Character Expressiosupporting

confidence: 69%

Section: Implications Ofaphelandra Hybrids For Phylogenetic Analysismentioning

confidence: 99%

Hybrids and Phylogenetic Systematics I. Patterns of Character Expression in Hybrids and Their Implications for Cladistic Analysis

McDade

1990

Evolution

115

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“…Although the occurrence of backcrossed individuals in hybrid swarms is well documented (e.g., Heiser, 1949;Alston andTurner, 1963, Levin, 1975;Soltis and Soltis, 1986), the I Present address: Rancho Santa Ana Botanic Garden, 1500 N. College, Claremont, CA 91711. permanent addition of genes from one species into another, resulting in the creation and establishment of a new type, has rarely been demonstrated (Heiser, 1973). Nevertheless, numerous studies have implicated introgression in the origin and establishment of new races, varieties, subspecies, and species (e.g., Heiser, 1949Heiser, , 1951bGrant, 1950;Levin, 1963;Ornduff, 1967;Bloom, 1976;Davis, 1985). Although introgression may have occurred in these examples, they are based primarily on morphological data and historical inference, making conclusive documentation of introgression difficult.…”

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A MOLECULAR REEXAMINATION OF INTROGRESSION BETWEEN HELIANTHUS ANNUUS AND H. BOLANDERI (COMPOSITAE)

1988

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Abstract. -Heiser (1949) hypothesized that a weedy race of Helianthus bolanderi had originated by the introgression of genes from H. annuus into a serpentine race of H. bolanderi. Although Heiser's investigation of these species is frequently cited as one ofthe best examples ofintrogression in plants, definitive evidence of gene exchange is lacking (Heiser, 1973). To determine whether the weedy race of H. bolanderi actually originated via introgression, we analyzed allozyme, chloroplast-DNA (cpDNA), and nuclear-ribosomal-DNA (rDNA) variation.Evidence from enzyme electrophoresis did not support the proposed introgressive origin ofweedy H. bolanderi. We detected a total of 37 low-frequency alleles distinguishing the serpentine race of H. bolanderi from H. annuus. Weedy H. bolanderi possessed only four of the 37 marker alleles. Further analysis demonstrated that serpentine H. bolanderi combined seven of the 35 alleles distinguishing H. annuus from weedy H. bolanderi, indicating that serpentine H. bolanderi shares three more marker alleles with H. annuus than does weedy H. bolanderi. These results are similar to expectations for race divergence from a single common ancestor and suggest that, ifintrogression occurred, the majority of marker alleles were rapidly lost following the initial hybridization event.Even more compelling evidence opposing Heiser's (1949) hypothesis, however, was from restriction-fragment analysis of cpDNA and nuclear rDNA. We detected a total of 17 cpDNA and five rDNA restriction-site mutations among the 19 populations examined. No parallel or back mutations were observed in phylogenetic trees constructed using eithercpDNA or rDNA mutations, and both phylogenies were completely congruent regarding the alignment of all three taxa. In addition, the weedy race of H. bolanderi possessed a unique cpDNA, which was outside the range of variation observed among populations of either ofthe presumed parental species. Mean sequence divergences between the cpDNAs of weedy H. bolanderi and those of serpentine H. bolanderi and H. annuus were 0.30% and 0.35%, respectively. These estimates are comparable to sequencedivergence values observed between closely related species in other plant groups. Given the lack of parallel or convergent mutations in the cpDNA and rDNA phylogenetic trees, the complete congruence of these trees with flavonoid-and allozyme-variation patterns, and the presence of a unique and divergent chloroplast genome in the weedy race of H. bolanderi, we suggest that the weedy race of H. bolanderi was not derived recently through introgression, as hypothesized, but is relatively ancient in origin.

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Hybrids and Phylogenetic Systematics Ii. The Impact of Hybrids on Cladistic Analysis

McDade

1992

Evolution

170

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I examined three aspects of the cladistic treatment of a set of 17 F hybrids of known parental origin: (1) impact of hybrids on consistency index (CI) and number of most parsimonious trees (Trees), (2) placement of hybrids in cladograms, and (3) impact of hybrids on hypotheses of relationship among species. The hybrids were added singly and in randomly selected sets of two to five to a data set composed of Central American species of Aphelandra (including the parents of all hybrids). Compared to analyses with the same number of OTUs all of which were species, the analyses with hybrids yielded results with significantly higher CI. There was no difference in Trees between analyses with hybrids versus species. There was thus no evidence that hybrids would appear to be more problematic for cladistic methods than species. Accordingly, hybrids will not be readily identifiable as taxa that cause marked change in these indices. About % of the hybrids were placed as the cladistically basal members of the lineage that included the most apomorphic parent. Relatively apomorphic hybrids were placed proximate to the most derived parent (ca. 13% of hybrids). Other placements occurred more rarely. The most frequent placements of hybrids thus did not distinguish them from normal intermediate or apomorphic taxa. When analyses with hybrids yielded multiple most parsimonious trees, these were no more different from each other than were the equally parsimonious trees that resulted from analyses with species. Most analyses with one or two hybrids resulted in minor or no change in topology. When hybrids caused topological change, they frequently caused rearrangements of weakly supported portions of the cladogram that did not include their parents. When they disrupted the cladistic placement of their parents, they often caused their parents to change positions, with at least one topology bringing the parental lineages into closer proximity with the hybrid placed between them. Hybrids between parents from the two main lineages of the group caused total cladistic restructuring. In fact, the degree of relationship between a hybrid's parents (measured by both cladistic and patristic distance) was strongly correlated with CI (negatively) and with the degree of disturbance to cladistic relationships (positively). Thus, hybrids between distantly related parents resulted in cladograms with low CI and major topological changes. This study suggests that hybrids are unlikely to cause breakdown of cladistic structure unless they are between distantly related parents. However, these results also indicate that cladistics may not be specially useful in distinguishing hybrids from normal taxa. The applicability of these results to other kinds of hybrids is examined and the likely cladistic treatment of hybrids using other sources of data is discussed.

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Introgression in Central American Phytolacca (Phytolaccaceae)

Cited by 3 publications

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Hybrids and Phylogenetic Systematics I. Patterns of Character Expression in Hybrids and Their Implications for Cladistic Analysis

Hybrids and Phylogenetic Systematics I. Patterns of Character Expression in Hybrids and Their Implications for Cladistic Analysis

A MOLECULAR REEXAMINATION OF INTROGRESSION BETWEEN HELIANTHUS ANNUUS AND H. BOLANDERI (COMPOSITAE)

Hybrids and Phylogenetic Systematics Ii. The Impact of Hybrids on Cladistic Analysis

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