Eco-Geography and Phenology Are the Major Drivers of Reproductive Isolation in the Royal Irises, a Species Complex in the Course of Speciation

Abstract

The continuous nature of speciation implies that different species are found at different stages of divergence, from no- to complete reproductive isolation. This process and its underlying mechanisms are best viewed in incipient species. Moreover, the species complex can offer unique insight into how reproductive isolation (RI) has evolved. The royal irises (Iris section Oncocyclus) are a young group of species in the course of speciation, providing an ideal system for speciation study. We quantified pre- and post-zygotic reproductive barriers between the eight Israeli species of this complex and estimated the total RI among them. We tested for both pre-pollination and post-pollination reproductive barriers. Pre-pollination barriers, i.e., eco-geographic divergence and phenological differentiation were the major contributors to RI among the Iris species. On the other hand, post-pollination barriers, namely pollen–stigma interactions, fruit set, and seed viability had negligible contributions to total RI. The strength of RI was not uniform across the species complex, suggesting that species may have diverged at different rates. Overall, this study in a young, recently diverged group of species provides insight into the first steps of speciation, suggesting a crucial role of the pre-zygotic barriers.

Flower Size as an Honest Signal in Royal Irises (Iris Section Oncocyclus, Iridaceae)

“…The decrease in flower size of the seven Royal irises measured under controlled conditions matches their natural distribution from north to south (Figure 1, Supplementary Figure S6A), following the aridity gradient (for the natural occurrence of the species, see Figure 1 in Ref. [58]). Black patch size and tunnel volume did not exhibit such a decrease, and seem to be less variable among species (Supplementary Figure S6B,C), suggesting that the climate gradient does not affect these traits as much as it affects flower size.…”

“…We measured 59 flowers in total: I. petrana (n = 10), I. atropurpurea (n = 16), I. mariae (n = 19), I. haynei (n = 5), I. lortetii (n = 3), I. hermona (n = 3), and I. bismarckiana (n = 3). In their natural environment, these species are distributed along the north–south aridity gradient of Israel [ 34 ] and are eco-geographically isolated with different levels of pre- and post-zygotic reproductive barriers [ 58 , 61 ]. For an overview of the natural distributions of these species in Israel and Palestine, see Figure 1 in Ref.…”

“…For an overview of the natural distributions of these species in Israel and Palestine, see Figure 1 in Ref. [ 58 ].…”

Strong postmating reproductive isolation in Mimulus section Eunanus

“…By placing reproductive isolation and divergence in a phylogenomic context, species complexes allow us to make informed comparisons across groups, distinguishing patterns and trends while uncovering crucial differences. For example, the degree of reproductive isolation between two taxa tends to increase with genetic distance (Christie & Strauss, 2018; Coyne & Orr, 1989; Malone & Fontenot, 2008; Scopece et al, 2007), but careful work in irises and other species complexes has demonstrated substantial heterogeneity in this relationship (Moyle et al, 2004; Osmolovsky et al, 2022). In addition, the emergence speed of different categories of isolation (Christie & Strauss, 2018) or of specific reproductive barriers (e.g.…”