Revista Chilena de Historia Natural 68: 451-458, 1995 Variation of the architectural phenotype of aphyllus in central Chile

Variaci6n del fenotipo arquitectural de Tristerix aphyllus en Chile central

RODRIGO G. MEDEL, ELISEO VERGARA, ARTURO SILVA and IT ALO A. SEREY

Departamento de Ciencias Ecologicas, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile

ABSTRACT

We analyze the morphological variation of the holoparasitic Tristerix aphyllus in 20 populations of central Chile by focusing upon 8 qualitative characters that represent different architectural attributes of the parasite. Our results show that specific characters present a low congruent variation across populations, which suggests a low integration of the architectural phenotype. Similarly, 44 different combinations of traits were identified, and their prevalence at each population was indistinguishable from that occasioned by a random process. Characters did not contribute equally to the overall architectural variation. Sympodial growth pattern, tetrapodism, single branch, and closed branch were the most important traits in contributing to architectural variation across populations. The disintegrated architectural phenotype indicates absence of a unique architectural design in this species. Rather, the random prevalence of character combinations suggests that the geographical pattern may be better described as a "random patchwork". On the base of these antecedents, and considering the leafless holoparasitic biology ofT. aphyllus we suggest that non adaptive architecture prevails in this species. Key words: mistletoe, parasite, host, , Mediterranean habitat.

RESUMEN

Analizamos Ia variacion morfologica del muerdago holopanisito Tristerix aphyllus en 20 poblaciones de Chile central. Focalizamos en 8 caracteres cualitativos los cuales representan diferentes atributos arquitecturales del parasito. Los caracteres especificos presentan una baja variacion congruente entre poblaciones, lo cual sugiere una baja integracion del fenotipo arquitectural. De igual forma, se identificaron 44 combinaciones distintas de caracteres y su prevalencia en las poblaciones fue indistinguible de una ocasionada por azar. Los caracteres no contribuyeron de igual manera a Ia variacion arquitectural. Patrón de crecimiento simpodial, tetrapodismo, rama simple, y rama cerrada fueron las rasgos más importantes en contribuir a Ia variacion total. La desintegracion fenotfpica documentada en este estudio sugiere ausencia de un único disefio arquite<;tural en esta especie. Por el contrario, Ia prevalencia aleatoria de combinaciones de caracteres en las poblaciones sugiere que el patron geografico podrfa ser mejor descrito como un "mosaico aleatorio". Sobre Ia base de estos antecedentes, y considerando Ia biologfa holoparasita sin hojas de T. aphyllus, sugerimos que los rasgos arquitecturales que prevalecen en esta especie no conllevan valor adaptativo. Palabras clave: muerdago, parasito, hospedador, Loranthaceae, habitat mediterraneo.

INTRODUCTION constraints form and development, most studies dealing with plant architecture Architecture is an important but often have been conducted on non-parasitic neglected component of the plant phenotype. where vegetative and reproductive axes are Architectural design tend to be related with integrated into the same structure (Halle et nutrient absorption and light interception al. 1978, Kuuluvainen & Pukkala 1991). (Kiippers 1989, Room et al. 1994), and Situation with holoparasitic plants is less certain combinations of traits have provided clear because hosts can potentially provide to be optimal for increased nutrient uptake sufficient nutrients to make unnecessary (e.g., Lovell & Lovell 1985, Kiippers 1989). any single optimal architectural design in In spite of the potential importance of parasites. Hosts represent the more im- architecture to reveal the way environment mediate surrounding for the parasite (Herrera

(Received 24 December 1994; accepted 10 May 1995; managed by Gloria Montenegro) 452 MEDELET AL.

1988a, Hutchings & de Kroon 1994 ), and T. aphyllus vary uniformly or does it phenotypic evolution of some hemiparasitic represent a random patchwork with traits species has been demonstrated to be in- combining randomly across populations? fluenced by host species (e.g., Atsatt 1970a, Atsatt & Guldberg 1978). In spite of, studies dealing with the architecture of holoparasitic MATERIALS AND METHODS plants are lacking in the literature. This paper focuses on the architectural We sampled 20 populations ofT. aphyllus in design of Tristerix aphyllus, a Chilean en- central Chile. We selected 10 populations demic holoparasitic mistletoe that parasitizes from the Reserva Nacional Las Chinchillas a total of 17 species of cacti in the Chilean (31 o 30' S, 71 o 06' W), located 17 km north arid and semiarid zones (Follman & Mahú Illapel. Other 10 populations were selected 1964). Research on the architecture of T. around Til-Til (33° 28' S, 70° 52' W), located aphyllus is pertinent because several approximately 60 km north Santiago. Sampl- anatomic and physiological properties make ing consisted in collecting 40 this species an unusual mistletoe that each from a different individual of the cactus depends entirely on the host for growth and Echinopsis chilensis per population, taking reproduction (Kuijt 1969). For example, care of removing completely the reproduc- unlike most Loranthaceae, T. aphyllus has tive portion of mistletoes. Samples were leaves reduced to minute scales with only carried to the lab for analyses and examined the reproductive structure emerging from with a binocular microscope. We examined a the parasitized cacti. The rest of vegetative total of 800 individuals for architectural portion exists as an endophyte within the traits. All sampling was conducted during tissues of the cacti (Mauseth 1990). These March-May 1994, during the flowering characteristics have been interpreted as season ofT. aphyllus. anatomic adaptations to reduce potential We focused upon eight qualitative excessive water loss in Chilean arid zones characters related to the architectural design (Mauseth 1985, 1991). Although the mech- of the mistletoe (Figure 1) (Prusinkiewicz & anism of host entry and the associated Lindenmayer 1990): 1) Monopodia! pattern anatomical characteristics of T. aphyllus of growth, that refers to the situation when have been extensively studied by Mauseth the main apex continues growing but all et al. ( 1984, 1985), the morphology of its lateral apices stop growing. 2) Sympodial reproductive portion has not received similar pattern of growth, that occurs when the prin- attention. The only reference indicates stems cipal axe is occupied by a lateral branch that of its external portion are reddish and 5-20 gets apical dominance. 3) Tetrapodic branch, em long (Hoffmann 1978). In an extensive that refers to the formation of a verticile with review of the genus, Kuijt (1988) referred four branches with shortened internode to the morphology of Tristerix as "The distances. 4) Single branch, that refers to the branching pattern of all species seems to be formation of a unique lateral branch along basically the same, although the situation in the principal axis. 5) Double branch, that T. aphyllus needs more careful study." (p. 6). refers to the formation of two lateral The aim of this paper is to assess the branches with a minimal internode distance. extent to which the architectural phenotype 6) Closed branch, that refers to a relative of a holoparasitic mistletoe varies uniformly shortening of the internode distance among across populations as expected for an optimal alternate pairs of branches when branching is architectural design. This study addresses opposite along the principal axis. 7) Opposite the following questions. (1) Are populations branch, that refers to the formation of lateral of T. aphyllus homogeneous in architectural branches in an opposite form along the prin- traits?, (2) Do architectural traits vary in- cipal axis. 8) Alternate branch, that refers dependently across populations or are they to the formation of lateral branches in an organized into coordinate varying subsets? alternate form along the principal axis. The Responses to these questions are related to first two characters represent a unit of the more general one, (3) does architecture of analysis somewhat different from the ARCHITECTURAL PHENOTYPE OF TRISTERIX APHYLLUS 453

monopodic sympodic tetrapodic single (verticilate) branch

double closed opposite alternate branch branch branch branch Fig. 1: Graphical description of the architectural characters of T. aphyllus used in this study. Descripci6n gráfica de los caracteres arquitecturales de T. aphyllus usados en este estudio. remaining traits because of their higher applied a test for homogeneity to each pair importance to the overall pattern of growth. of coefficients by using the Hotteling' s Z2 Because abortion of a meristem can po- transformation after applying the Bonferroni tentially originate a monopodia! and/or a criterium to correct for the 28 multiple sympodial pattern (Callaghan et al. 1986, contrasts (Rice 1989, Sokal & Rohlf 1995). Mueller 1988), we searched for the two Previous analyses revealed that none of characters at each individual. the 800 individuals examined exhibited the Populations examined comprised two eight characters simultaneously, but 44 dif- distant localities ( 10 populations from Las ferent trait combinations were detected. In Chinchillas and 10 around Til-Til). Our first order to know the traits that contribute most analysis consisted in assessing for potential to the total architectural variation, we clas- differences in architectural traits among the sified every individual according to its parti- two groups of populations. We calculated the cular trait combination and proceeded to percentage of mistletoes exhibiting each calculate an index of population architectural character at each population and assessed for variation. We used the inverse of Simpson's global differences among the two localities index, B = ¹/Ʃpi², where pi represents the by discriminant analysis on the multivariate population frequency of individuals with architectural phenotype (Hair et al. 1992). architectural combination i. With this clas- We next evaluated if pairs of traits exhibited sification we evaluated the contribution of congruent variation across populations. We each trait to the total architectural variability constructed correlation matrixes using every by using stepwise regression with no varia- character pair at each of the two localities bles forced into the models and P values for and tested for heterogeneity of correlation partial F tests of 0.05 to enter and 0.10 to coefficients among the two situations. We remove. Because characters were present in 454 MEDELET AL. more than one combination, they may not F (8,11) = 9.72, P = 0.001, MANOVA). Only affect independently the architectural varia- the first of the eight canonical axes revealed tion of T. aphyllus. To evaluate such pos- significant discrimination (83.2% of the sibility, we tested for correlated causes of differences among localities). The different characters on architectural variability, B, by signs of some coefficients of the canonical means of path analysis. discriminant function indicate that antagonic In order to assess for coordinate variation characters related to the apical dominance of subsets of characters across populations processes (monopodism and sympodism) we calculated the similarity in the trait com- and antagonic characters related to formation binations among pairs of populations by of lateral branches (opposite and alternate means of the Jaccard's similarity index. Be- branch) were important in discriminating cause similarity may not differ from a among localities (Table 2). However, random sampling of combinations, we comparison of the structure of character constructed a bootstrap algorithm (Efron correlation matrixes did not reveal sig- 1981, Manly 1991), that consisted in the nificant differences among localities. This random assignation of trait combinations suggests that the determinants of architec- to each population. The selected combination tural integration in T. aphyllus have not was returned each time to the original data- undergone significant differentiation among base up to completing the real number of localities. combinations in the population. In this Most architectural characters tended to way, it was possible to compare the observed vary independently across populations ex- distribution of trait combinations in the 20 cepting six pairs that associated congruently populations with a distribution based on a in populations near Til-Til. None of such random sampling process. Excepting the pairs exhibited significant association in randomization procedure, all analyses were Las Chinchillas. The only consistent trend in carried out by using SYSTAT statistical trait covariation was the pair monopodia! package (Wilkinson 1992). growth-opposite branch, which showed a significant positive relationship in popula- tions of the two localities (Table 3). When RESULTS AND DISCUSSION the two correlation matrixes were tested for homogeneity, the null hypothesis of We detected significant differences in four homogeneity of correlation coefficients of the eight characters examined for could not be rejected, which imply that variation, which tended to be more frequent architectural characters do not associate in in the study sites near Til-Til (Table 1). different ways in the two localities. Con- When all characters are considered together, sequently, it is possible to conclude that the the multivariate architectural space was components of the architectural phenotype affected by locality (Wilks' lambda= 0.124, of T. aphyllus do not present congruent

TABLE I

Frequency of architectural characters of T. aphyllus in 20 populations of Las Chinchillas and Til-Til. Figures indicate mean± 2 SE Frecuencia de caracteres arquitecturales de T. aphyllus en 20 poblaciones de Las Chinchillas y Til-Til. Los valores indican media± 2 EE

Character Las Chinchillas Til-Til df F p monopodism ± ± 1,18 sympodism ± ± 1,18 tetrapodism ± ± 1,18 2.44 single branch ± ± 1,18 22.76 < double branch ± ± 1,18 31.59 < closed branch ± ± 1,18 11.13 opposite branch ± ± 1,18 9.11 alternate branch ± ± 1,18 ARCHITECTURAL PHENOTYPE OF TRISTERIX APHYLLUS 455

TABLE2 contributing significantly to architectural variation (first entry: sympodial growth P < Coefficients of the standardized canonical discriminant function in the 0.001; second entry: tetrapodism P < 0.001; multivariate comparison of architecture third entry: closed branch P < 0.05; fourth of T. aphyllus among localities entry: single branch P < 0.05). The analysis Coeficientes de Ia funci6n discriminante of independent causes measured by Pearson's can6nica estandarizada en el contraste multivariado correlation analysis revealed significant de Ia arquitectura de T. aphyllus entre localidades coefficients when each trait was correlated

Character First canonical axe with B. Path analysis revealed correlated correlation coefficients similar to Pearson's Monopodism -0.050 independent coefficients (sympodial growth Sympodism 0.235 Tetrapodism 0.163 = 0.853, tetrapodism = 0.755, single branch Single branch 0.462 = 0.683, closed branch = 0.603). This result Double branch 0.387 implies that correlation with B is occasioned Closed branch 0.482 Opposite branch 0.366 not only by the direct contribution of indivi- Alternate branch -0.135 dual characters but also by interactions among them. This is graphically depicted in Figure 2. variation but tend to vary independently Similarity values among populations in across populations. the types of characters combinations were When combinations of characters rather low (mean ± 2 SE: 0.14 ± 0.21), and com- than specific traits were analyzed, a total of parison with values expected from a random 44 different combinations was observed in process revealed non significant differences the 20 populations which occasioned values (Student's test, P > 0.16). This result implies of variability, B, ranging from 2.18 to 8.69. that the subsets of architectural characters do Contribution of specific characters to archi- not tend to vary coordinately across pop- tectural variation was assessed by stepwise ulations which further confirms the previous regression. Four characters were selected as finding of absence of congruent variation of

TABLE 3

Correlation matrix for congruent variation of characters ofT. aphyllus. LC =Las Chinchillas, TT = Til-Til, Mono = monopodism, Symp = Sympodism, Tetr = tetrapodism, Sbra = single branch, Dbra = double branch, Cbra = closed branch, Obra = opposite branch . *p < 0.05, ** p < 0.01, *** p < 0.001 Matriz de correlaci6n para variaci6n congruente de caracteres de T. aphyllus. LC = Las Chinchillas, TT = Til-Til, Mono= monopodismo, Symp = simpodismo, Tetr = tetrapodismo, Sbra = rama nica, Dbra = rama doble, Cbra = rama cerrada, Obra = rama opuesta. 'P < 0.05, ** P < 0.01, *** P < 0.001

Mono Symp Tetr Sbra Dbra Cbra Obra

Sympodism LC 0.56 -0.16 TT -0.16 Tetrapodism LC -0.41 -0.05 TT -0.70* 0.54 Single branch LC -0.19 0.23 -0.05 TT -0,67' 0.56 0.648 Double branch LC -0.07 -0.50 0.06 -0.17 TT -0.36 0.26 0.49 0.70* Closed branch LC 0.44 0.31 -0.26 0.37 -0.30 TT -0.05 0.27 0.29 -0.19 -0.41 Opposite branch LC 0.82** 0.37 -0.52 0 -0.25 0.49 TT 1.00*** -0.16 -0.70' -0,67' -0.36 -0.05 Alternate branch LC 0.41 0.19 -0.07 0.10 0.33 0.37 -0.03 TT 0.20 0.07 0.01 0.11 -0.06 0.31 0.20 456 MEDELET AL.

BRANCH

BRANCH Fig. 2: Path diagram illustrating the architectural characters that correlate with the overall architectural variation. Figures over straight lines pointing to the architectural variation indicate standard partial regression coefficients. Figures over lines connecting architectural characters indicate Pearson's correlation coefficients. * P < 0.05, *** P < 0.001. Diagrama de vias que ilustra los caracteres arquitecturales que se correlacionan con Ia variaci6n arquitectural. Los valores sobre las lineas rectas apuntando hacia variaci6n arquitectural indican coeficientes de regresi6n parcial estandar. Los valores sobre las lineas que conectan los caracteres arquitecturales indican coeficientes de correlaci6n de Pearson. * P < 0.05, *** P < 0.001. specific characters across populations. On pollinator and disperser assemblages across the base of these results it is possible to populations. conclude that different architectural pheno- The high architectural variation document- types of T. aphyllus appear randomly across ed in this paper may be attributed, at least in populations. part, to the condition of holoparasite favoring The absence of covariation in individual phenotypical canalization by hosts (Atsatt characters at each locality and the high 1970a). Independent variation among pop- number of different character combinations ulations may stem from genetic differences, found among populations suggests that the host age differences and variation in host pattern of architectural morphology may physiological and nutrient status, or most be better described as a "random patchwork", likely a combination of these. For instance, with frequencies of characters and character high phenotypical variation in morphological combinations occurring randomly across and reproductive traits is a conspicuous populations. The high architectural diversity feature of hemiparasites (Smith 1963, Bobear found in this study suggests absence of a 1969, Atsatt 1970a,b, Atsatt & Guldberg single design for T. aphyllus, thus question- 1978, Herrera 1988b) that tend to be ing the existence of adaptive architecture in correlated with increased genetic diversity this species. Mechanisms responsible for the (Atsatt 1970a, Atsatt & Strong 1970, Atsatt high architectural variability of T. aphyllus & Guldberg 1978). Although the genetic are unknown. Biotic factors such as the basis of architectural design in T. aphyllus is identity of pollinators and fruit dispersers unknown, the factors that determine the species may contribute to increase variability phenotypical variation in this species are among plant populations. However, T. probably different from those concerning aphyllus is pollinated almost exclusively by land autotrophic or even hemiparasitic plants the hummingbird Sephanoides galleritus (Atsatt 1983, Aparicio 1993). and its fruits are dispersed uniquely by The high diversity of architectural pheno- the Chilean mockingbird Mimus thenca. types within populations suggests that the Although changes in the density of pollinator physiological and nutrient status of hosts and disperser species may eventually affect may be more important than external en- the architecture of this holoparasite, it is vironmental factors in determining the unlikely that architectural variation among parasite phenotype. It is possible that nutrient populations is occasioned by changes in absorption from external sources and light ARCHITECTURAL PHENOTYPE OF TRISTERIX APHYLLUS 457 interception do not represent important ATSATT PR (1983) Mistletoe leaf shape: a host morphogen hypothesis. 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