A Trade-Off Between Sporangia Size and Number Exists in the Potato Late Blight Pathogen Phytophthora infestans, and Is Not Altered by Biotic and Abiotic Factors

Abstract

The negative relationship between offspring size and number is a classic example of trade-off between life-history traits, reported many times in animal and plant species. Here, we wanted to ascertain whether such a trade-off occurred in the oomycete Phytophthora infestans, and whether it was impacted by biotic and abiotic factors. We thus conducted three infection experiments under controlled conditions and measured the number and the size of sporangia (asexual propagules) produced on potato by different P. infestans isolates. In all experiments, we observed a negative relationship between sporangia size and number, demonstrating the existence of a trade-off. Moreover, although the potato host cultivar, temperature and host of origin (tomato or potato) all affected sporangia number, sporangia size or both, none of these biotic and abiotic factors did change the trade-off. Therefore, the trade-off between sporangia size and number could maintain the polyphenism for these traits in P. infestans populations, and favors the coexistence of distinct reproductive strategies within this species. Our results emphasize the relevance to focus on the relationship between offspring size and number in other fungal plant pathogens, as well as to study the impact of offspring size on fitness-linked traits (virulence and disease lesion development) in these organisms.

Keywords: Solanum tuberosum; fitness; life-history traits; plant pathogen; reproductive strategies.

FIGURE 1

Relationship between size and number of sporangia produced by 119 P. infestans isolates after inoculations on potato cultivars Bintje, Möwe, and Robijn (experiment 1). All variables were LOG-transformed. For sporangia produced on Bintje, the linear regression is represented by a continuous gray line (y = 10.89–0.14×, R² = 0.13, P < 0.001), for sporangia produced on Möwe, the linear regression is represented by a continuous black line (y = 10.58–0.13×, R² = 0.23, P < 0.001), and for isolates inoculated on Robijn, the linear regression is represented by a dotted black line (y = 10.22–0.10×, R² = 0.24, P < 0.001).

FIGURE 2

Relationship between size and number of sporangia produced by 16 P. infestans isolates after inoculations on potato cultivar Bintje at 10, 14, and 18°C (experiment 2). All variables were LOG-transformed. For sporangia produced at 10°C, the linear regression is represented by a continuous gray line (y = 11.25–0.21×, R² = 0.49, P < 0.001), for sporangia produced at 14°C, the linear regression is represented by a dotted black line (y = 11.58–0.23×, R² = 0.39, P < 0.001) and for isolates inoculated at 18°C, the linear regression is represented by a continuous black line (y = 11.14–0.21×, R² = 0.46, P < 0.001).

FIGURE 3

Relationship between size and number of sporangia produced by 21 P. infestans isolates sampled on potato or tomato after inoculations on potato cultivar Bintje (experiment 3). All variables were LOG-transformed. For sporangia produced by potato isolates, the linear regression is represented by a continuous black line (y = 10.23–0.10×, R² = 0.32, P = 0.006) and for sporangia produced by tomato isolates, the linear regression is represented by a continuous gray line (y = 10.20–0.11×, R² = 0.34, P = 0.007).