Janzen-Connell effects shape gene flow patterns and realized fitness in the tropical dioecious tree Spondias purpurea (ANACARDIACEAE)

Sex-specific outbreeding advantages and sexual dimorphism in the seedlings of dioecious trees

PREMISE

Dioecious trees are important components of many forest ecosystems. Outbreeding advantage and sexual dimorphism are two major mechanisms that explain the persistence of dioecious plants; however, they have rarely been studied in dioecious trees.

METHODS

We investigated the influence of sex and genetic distance between parental trees (GDPT) on the growth and functional traits of multiple seedlings of a dioecious tree, Diospyros morrisiana.

RESULTS

We found significant positive relationships between GDPT and seedling sizes and tissue density. However, the positive outbreeding effects on seedling growth mainly manifested in female seedlings, but were not prominent in males. Among seedlings, the male ones generally had higher biomass and leaf area than female seedlings, but such differences diminished as GDPT increased.

CONCLUSIONS

Our research highlights that outbreeding advantage in plants can be sex-specific and that sexual dimorphism begins from the seedling stage of dioecious trees.

Genetic diversity, asexual reproduction and conservation of the edible fruit tree Spondias purpurea L. (Anacardiaceae) in the Costa Rican tropical dry forest

The term circa situm has been used to describe different conservation strategies within agricultural landscapes. Circa situm conserves planted or remnant species in farmlands, where natural vegetation has been modified through anthropogenic intervention. It has been proposed that trees planted or retained under circa situm conditions may contribute to maintaining genetic diversity, however information on the role of this strategy in preserving genetic diversity is scarce. The aim of this study was to determine the levels of genetic diversity and structure, and mating patterns in planted and unmanaged stands of the tropical fruit tree Spondias purpurea L. in north western Costa Rica. In three localities, we used seven polymorphic microsatellite loci and genotyped 201 adults and 648 seeds from planted and wild stands. We found no differences in genetic diversity among planted and wild stands. Genetic structure analysis revealed that gene flow occurs among planted and wild stands within localities. Clones were present and their diversity and evenness were both high and similar between planted and wild stands. The number of pollen donors per progeny array was low (Nep = 1.01) which resulted in high levels of correlated paternity (rp = 0.9). Asexual seeds were found in 4.6% of the progeny arrays, which had multilocus genotypes that were identical to the maternal trees. Our results show that although planted stands under circa situm conditions can maintain similar levels of genetic diversity than wild stands, the low number of sires and asexual seed formation could threaten the long term persistence of populations.

Growing Out of the Tropical Forests: Gene Flow of Native Mesoamerican Trees Among Forest and Mayan Homegardens

This work aimed to evaluate domestication effects on the genetic structure of two dioecious speciesBrosimum alicastrumSw. (Moraceae) andSpondias purpureaL. (Anacardiaceae), and a heterostylous oneCordia dodecandraA. DC. (Cordiaceae), growing in remnant forests and homegardens within two climatic regions of the Peninsula of Yucatan. The trees ofB. alicastrumandC. dodecandraare propagated by seeds in both population types, while those ofS. purpureaare propagated asexually in the homegardens. ISSRs genetic markers were amplified from foliar tissue of 18 to 21 plants per population type/region combination for each species. Genetic diversity, genetic differentiation, and genetic structure estimators were obtained and compared among species at the regional and population level. We found higher polymorphism (37.5–41), but lower private alleles (4–4.4) and similar heterozygosity (0.1–0.12) in the species with sexual reproduction compared toS. purpurea(34, 8, and 0.11, respectively). Genetic diversity inB. alicastrumpopulations varied with the region; inC. dodecandra, to the population type; and inS. purpurea, to both the population type and the region. Unrestricted gene flow among regions was suggested by low ΦRTinC. dodecandraandS. purpurea(−0.006 and 0.002) but not forB. alicastrum(0.1). Gene flow between populations within the regions for the sexually reproducing species was suggested by lower θII(0.005–0.07 and 0.008–0.1) estimates than those ofS. purpurea(0.09 and 0.13). Even though the lowest paired FST(0.002–0.05) and ΦST(0.002–0.12) values were found between the northeastern forest and homegarden populations for the three species, the dendrogram, Bayesian assignment, and K-Means analyses suggest that the least differentiated populations are southwestern forest and homegarden populations ofB. alicastrumandS. purpurea, and the southwestern forest and northeastern homegarden ofC. dodecandra. The sexual reproduction, biotic interactions, and extensive management ofB. alicastrumandC. dodecandrain the agroforestry and the urban systems may contribute to connectivity between wild and domesticated populations, while inS. purpureathis connectivity is interrupted by the clonal propagation of the species in the homegardens.