Economy, efficiency, and the evolution of pollen tube growth rates

Abstract

We showed that growth rate efficiencies evolved by changes in the volume of wall material used for growth and in how that material was partitioned between lateral and length dimensions. The economics of pollen tube growth are determined by tube design, which is consequent on trade-offs between efficient growth and other pollen tube functions.

How does genome size affect the evolution of pollen tube growth rate, a haploid performance trait?

“…First, Mulcahy () invoked a shift to much higher intensity of pollen competition in angiosperms as a driver of the origin and continued evolution of faster growth rates. Notably, no other type of tip‐growing cell in land plants (whether gametophytic or sporophytic) has evolved comparably fast tip‐growth rates and none of those cell types, including gymnosperm pollen tubes, experience intense competition for resources (Williams et al., ). Secondly, gymnosperm PTGR s may be slow because they lack novel biophysical or physiological attributes of pollen tubes and/or novel attributes enabled faster PTGR s to evolve in angiosperms (Hoekstra, ; Derksen et al., ; Fernando et al., ; Williams, , ).…”

“…Nucleotypic effects on PTGR could be substantial. Tube size affects PTGR in a linear fashion, because larger tubes must make more tube wall per unit time, and since tube diameter is constant during growth, the rate of wall production is directly proportional to tip extension rate (Williams et al., ). Kostoff and Prokofieva () reported in vivo pollen tubes to be 39% larger in diameter in an allotetraploid Nicotiana (Solanaceae) relative to the mean of its presumed diploid progenitors, and Iyengar () found 8–53% larger tube diameters in tetraploid versus diploid species of Gossypium (Malvaceae).…”

“…It has been argued that their pecto‐cellulosic wall structure is a limitation relative to angiosperm pollen tube walls, which use the plasma membrane‐bound enzymes callose synthase and pectin‐methylesterase in a novel way to more rapidly synthesize a strong and durable tube cell wall and callose plugs (Derksen, ; Abercrombie et al., ; Wallace and Williams, ). However, other types of pecto‐cellosic tip‐growing cells, such as root hairs, grow faster than gymnosperm pollen tubes (Williams et al., ). Thus, it seems likely that the extremely slow growth rates of gymnosperm pollen tubes reflect an ancestrally antagonistic relationship between maternal tissues and pollen tubes that functioned as invasively growing rhizoids, coupled with a lack of selection for faster growth rate due to very weak pollen competition and a long period between pollination and fertilization.…”

How does genome size affect the evolution of pollen tube growth rate, a haploid performance trait?

“…Nucleotypic effects on PTGR could be substantial. Tube size affects PTGR in a linear 371 fashion, because larger tubes must make more tube wall per unit time, and since tube diameter is 372 constant during growth, the rate of wall production is directly proportional to tip extension rate 373 (Williams et al, 2016). Kostoff & Prokofieva (1935) reported in vivo pollen tubes to be 39% 374 larger in diameter in an allotetraploid Nicotiana relative to the mean of its presumed diploid 375 progenitors, and Iyengar (1938) found 8-53% larger tube diameters in tetraploid versus diploid 376 species of Gossypium.…”

“…First, Mulcahy (1979) invoked a shift to much higher intensity of 321 pollen competition in angiosperms as a driver of the origin and continued evolution of faster 322 growth rates. Notably, no other type of tip-growing cell in land plants (whether gametophytic or 323 sporophytic) has evolved comparably fast tip-growth rates and none of those cell types, including 324 gymnosperm pollen tubes, experience intense competition for resources (Williams et al, 2016). 325…”