Dominant Occurrence of Cleistogamous Flowers of Lamium amplexicaule in relation to the Nearby Presence of an Alien Congener L. purpureum (Lamiaceae)

Here we document a novel phenomenon that, based on field observations in central Japan, cleistogamous flowers (or closed flowers) of an annual herb Lamium amplexicaule were dominantly expressed near an alien congener L. purpureum. The proportion of cleistogamous flowers in an individual L. amplexicaule increased with the frequency of L. purpureum occurring in the same patches but did not increase with the total density of Lamium plants and their own size. To confirm the consistency of the effect of the coexisting alien species, we assessed the cleistogamous frequency at the patch level for three other populations. In these populations as well, the proportion of L. amplexicaule producing cleistogamous flowers increased with the frequency of L. purpureum. Our transplant experiment at one site found no effect of the nearby presence of L. purpureum on the seed set of L. amplexicaule and therefore did not support the hypothesis that the adverse effect on the reproduction via interspecific pollination favored cleistogamous flowers that accepted no external pollen. Further studies must be conducted to examine the negative interactions between the related species before and after seed development. 1. Introduction A cleistogamous (CL) breeding system is a selfing strategy that evolved independently in diverse plant taxa [1]. Cleistogamous flowers possess a reduced structure of nectar or petals and are obligatory self-fertilized without being open, whereas chasmogamous (CH) flowers can be cross-pollinated by another individual with an ordinal flower structure [2]. The cleistogamous breeding system has been regarded as a bet-hedging strategy for unpredictable environments [3–5], in which selfing by producing cleistogamous flowers can ensure reproductive success even in the absence of pollinators [6]. As another function of CL flowers, cost saving in producing flowers has been considered [5, 7] because the CL flowers are usually smaller and require less biomass than CH flowers. Empirically, smaller plants have been known to produce more CL flowers [8, 9]. Consequently, a few different hypotheses have been advanced for the function of the CL flowers. All have been in intraspecific contexts. The cleistogamous breeding system can be favored by interspecific interaction although it has been discussed in few reports to date [10–12]. For instance, deposition of hetero-specific pollens on stigmas has been known to reduce fruit set in some plants [11, 13, 14]. Such an interspecific interaction in the pollination is a typical mechanism of the reproductive interference,