In the male dimorphic mite Sancassania berlesei, fighter males kill rivals with a pair of armoured legs whereas scrambler males are benign with unmodified legs. In an adaptive response mediated by colony pheromones, fighter expression increases at low colony density. Under the status-dependent evolutionarily stable strategy (ESS) model we expected heavier final instar nymphs to become fighters. This was supported in group reared nymphs. In individually reared nymphs fighter expression was experimentally suppressed using two concentrations of colony pheromone. Here, male morph expression again depended on tritonymphal body mass and contact is therefore unnecessary for individuals to judge their status. Fighter suppression was greater in the higher pheromone treatment, but morph determination remained status dependent. The weight and length of fighters was lower than scramblers of same-weight final instar nymphs, indicating a developmental trade-off, and a cost not recouped at the adult stage.

Under the ‘good genes’ mechanism of sexual selection (SS), females benefit from mate choice indirectly: their offspring inherit genes of the preferred, high quality fathers. Recent models assume that the genetic variance for male quality is maintained by deleterious mutations. Consequently, SS can be predicted to remove deleterious mutations from populations. We tested this prediction by relaxing selection in populations of the bulb mite, thus increasing their rate of accumulation of deleterious mutation. SS, allowed to operate in half of these populations, did not prevent the fitness decline observed in the other half of the relaxed selection lines. After 11 generations of relaxed selection, female fecundity in lines in which males were allowed to compete for females declined compared with control populations by similar amount as in monogamous lines (17.5 and 14.5%, respectively), whereas other fitness components (viability, longevity, male reproductive success) did not differ significantly between both types of lines and control populations.

Considerable theoretical and empirical effort has been focused on the potential of continuously variable sexual traits to honestly indicate male quality, but relatively little effort has been devoted to a similar evolutionary role for dimorphic traits. Male dimorphisms, associated with conditionally expressed alternative reproductive tactics, represent extreme phenotypic plasticity. Evidence suggests that considerable heritable variation exists in the 'liability' underlying many threshold traits; if this liability is correlated with the genetic quality of males, dimorphic traits have the potential to be reliable indicators. We investigated the genetic architecture of phenotypically plastic morph expression in the context of condition-dependent signalling theory. Male morph in the mite Sancassania berlesei is condition dependent: 'fighters' armed with thickened and sharp third pairs of legs emerge from heavier nymphs than unarmoured 'scramblers'. We selected on male morph in three replicate 'fighter' and 'scrambler' lines and recorded a significant response to selection over seven generations; this was due to a shift in the threshold reaction norm but the lines showed no correlated response in condition. This is inconsistent with models predicting a substantial genetic correlation between condition and sexual trait expression. We discuss why dimorphic sexual traits may show more condition-independent genetic variance than continuous sexual traits.

The conditional evolutionarily stable strategy (ESS) with status‐dependent tactics is the most commonly invoked ESS for alternative reproductive tactics within the sexes. Support for this model has recently been criticized as apparent rather than real. We address key predictions of the status‐dependent ESS in three populations of the male dimorphic mite Sancassania berlesei. In S. berlesei‘fighter’ males are characterized by a thickened pair of legs used for killing rivals; ‘scramblers’ are benign. Most males in each population could be manipulated to become fighters by decreasing density, fulfilling the prediction that males make a ‘decision’. There was evidence of genetic covariance between sire status and offspring morph, but also a strong effect of sire morph on offspring morph ratio. This was consistent with considerable genetic variation for the status‐dependent switch point as a breeding experiment found no support for single‐locus inheritance. We also found evidence that switch points evolve independently of distributions of status. This study supports the current status‐dependent ESS model.

Research on the development of arbuscular fungi within thalli of Conocephalum conicum and Pellia endiviifolia was carried out on the banks of a stream ravine in the Beskid Wyspowy Mts. (Southern Poland). The links via arbuscular fungi were observed between liverworts and plants as Dryopteris carthusiana and Oxalis acetosella. Glomus tenuis, a fine endophyte, was the dominating mycorrhizal partner of all the plant species investigated. Arum-type of mycorrhiza was observed in Oxalis acetosella while in the fern and liverworts Paris-type was found. The role of plant roots in the establishment of liverworts thalli (source of fungal inoculum, important mechanical support on unstable sand banks) is considered