Distance-dependence seed set of Vasconcellea chilensis

Plant reproductive failure is a critical concern for conserving rare and endangered species that typically have low-density and sparse populations. One important factor contributing to reproductive failure is the spatial arrangement of plants within a population, which can lead to isolation and negatively affect seed output, particularly in obligate outcrosses. Additionally, this effect can be compounded by plant size. Here, we investigate how plants' spatial distribution and size influence the reproductive success of Vasconcellea chilensis, a threatened papaya species from northern Chile. Using flower exclusion experiments, we first examined whether V. chilensis can produce seeds via apomixis. We then used Spatial Point Pattern Analysis (SPPA) in three populations to examine the spatial arrangement of plants in three populations, and, finally, we assessed whether plant size and mate distance influence the reproductive success of this plant species. V. chilensis is a dioecious shrub unable to produce fruits through apomixis. The SPPA revealed significant clustering of female and male plants at different spatial scales, indicating a non-random distribution. Moreover, a significant attraction between the sexes suggested a preference for proximity. In two populations, closer proximity to male plants was linked to higher seed production. Our study revealed that the absence of apomixis in V. chilensis makes it prone to experiencing distance-dependent reproductive failure. In particular, the seed set was compromised in female plants isolated from male neighbors. This link between isolation and seed production was especially significant in the driest site, and we discussed how environmental factors can exacerbate this effect.

Methods
Vasconcellea chilensis (Planch. Ex A. Dc.) Solms 1889 (Caricaceae). Dioecious shrub with pistillate flowers (female) and staminate flowers (male).

Population Surveyed: Conchillas (CCH), Conchalí (CNL), and Puntilla Las Vacas (PLV).

Apomixis: We excluded pistillate flowers with mesh bags and marked untreated flowers as controls. After five to six months, we collected fruits and counted their seeds. 

Seed set: We collected five fruits from all female plants located in each population. For each plant, we calculate the seed set by dividing the mean number of seeds (in five fruits) by the mean number of ovules (in five flowers). 

Spatial Analyses: Individual plants were geo-referenced in the three populations. In each population, we run Spatial Point Pattern Analyses (SPPA) using Programita software. First, we calculate a univariate pair-correlation function (g(r)) separately for each sex, and second, we calculate a bivariate pair-correlation function (g12(r)) among both sexes (see details below).