Sexual modulation in a polyploid grass: a reproductive contest between environmentally inducible sexual and genetically dominant apomictic pathways

Abstract

In systems alternating between sexual and asexual reproduction, sex increases under unfavorable environmental conditions. In plants producing sexual and asexual (apomictic) seeds, studies on the influence of environmental factors on sex are equivocal. We used Paspalum intermedium to study environmental effects on the expression of sexual and apomictic developments, and on resulting reproductive fitness variables. Flow cytometric and embryological analyses were performed to characterize ploidy and reproductive modes, and effects of local climatic conditions on sexual and apomictic ovule and seed frequencies were determined. Seed set and germination data were collected and used to estimate reproductive fitness. Frequencies of sexual and apomictic ovules and seeds were highly variable within and among populations. Apomictic development exhibited higher competitive ability but lower overall fitness. Frequencies of sexual reproduction in facultative apomictic plants increased at lower temperatures and wider mean diurnal temperature ranges. We identified a two-fold higher fitness advantage of sexuality and a Tug of War between factors intrinsic to apomixis and environmental stressors promoting sexuality which influence the distribution of sex in apomictic populations. This points toward a crucial role of local ecological conditions in promoting a reshuffling of genetic variability that may be shaping the adaptative landscape in apomictic P. intermedium plants.

Figure 1

Reproductive analyses in tetraploid Paspalum intermedium plants. (a) Microscopic image of an ovule carrying a single MES; (b) Flow Cytometry histogram of sexual seed having a diploid embryo peak (2C) and a triploid endosperm peak (3C); (c) microscopic image of an ovule carrying one MES (red) and two AES (blue), with different spatial (MES sited toward the micropyle) and anatomical (AES lack antipodal cells) features; (d) Flow Cytometry histogram of an apomictic seed having a diploid embryo peak (2C) and a pentaploid endosperm peak (5C). ac: antipodal cells; ec: egg cell; pn: polar nuclei in the central cell. The bar represents 50 μm.

Figure 2

Plot showing the GLM fitted value lines of apomictic percentages of both embryo sacs and seeds in all the studied P. intermedium populations.

Figure 3

Map depicting the geographic variation of meiotic and apomictic ES percentages in different P. intermedium populations plotted against the MDR zones (contour lines demarcate variation zones; temperature ranges are given in Celsius). Red pies represent meiotic proportions; blue pies represent apomictic proportions.

Figure 4

Modelled distribution of reproductive pathway proportions at seed stages. Values of sexuality and apomixis gathered from natural populations are in black. Model projections of predicted mean values for sexuality and apomixis are depicted as red and blue dotted lines, respectively. Values of sexuality and apomixis from common garden experiments corresponding to the MDR zone of 11.1 °C ± 0.2 are in color.