Mental gland

Fig. 3.2 Amphibian protein-chemosignalling PRF (plethodontid receptivity factor interleukin-6), broadcast from male salamander mental gland in the forward current produced by tail-fanning (after Arnold, 1997 Rollman, 1999).

The chemoreceptive mechanisms in amphibia are undoubtedly worthy of further analysis, not only for their own sake, but to provide clues as to the origination of advanced chemosignal systems. As noted above, a pheromonal signal from the mental gland acts as a courtship/ receptivity inducer. The plethodontid receptivity factor (PRF) (Chap. 3) despite its size (22 kD), seems to have been converted from its internal role as an inter-cellular cytokine, to an inter-individual coordinator of reproductive activity (Rollmann et al., 1999). Endocrine or... 

Fig. 20.2 Pheromone delivery in Desmognathus ocoee salamanders and the method of pheromone delivery used during behavioural trials. A receptive female places her chin on the tail base of the male and typically straddles his tail. The male turns back towards the female and places his submandibular mental gland on her dorsum. The male then uses his premaxillary teeth to scratch the site on her dorsum that he has swabbed with his mental gland secretions. To mimic pheromone delivery in behavioural trials, each male was deglanded and a treatment solution was delivered to each female in a treatment patch (TrP) placed on her dorsum just posterior to the head. Photograph by Stevan J. Arnold...

Males were anesthetised and mental glands (N 200) were surgically removed. Secreted components were extracted into 0.8 mM acetylcholine chloride in l/2x PBS (cf Rollmann, Houck and Feldhoff 1999). Gland extracts were centrifuged for 10 min at 14,000 g and the supernatant was removed. The supernatant was filtered (0.2 pm) and loaded as aliquots onto a Sephadex Superfine G-75 gel filtration column (1.6 cm x 15.5 cm Pharmacia, Piscataway, NJ) on a Waters HPLC system (Mil-lipore, Milford, MA). The column had previously been equilibrated with one-half strength Dulbecco s phosphate buffered saline (l/2x PBS). The column was eluted... 

Fig. 20.4 Comparison of secreted proteins in pheromone extracted (A) from mental glands of Desmognathus ocoee (which has non-olfactory pheromone delivery) with (B) pheromone extracted from Plethodon shermani (which has olfactory delivery). Lanes 1 and 3 are MW standards. SDS-PAGE (15% Tris-Tricine)... 

Compared with the protein content of mental gland secretions of P. shermani males, the protein profile for D. ocoee showed a much more complex pattern of protein staining (Fig. 20.4). In D. ocoee, no single dominant protein occurred in this nonolfactory pheromone profile. 

Multiple lines of evidence suggest that SPF, a single protein within the 20-25 kDa pheromone fraction, is responsible for female behavioural response. First, this major protein component within the D. ocoee fraction was genetically very similar to the precursor of sodefrin (Palmer, Watts, Houck, Picard and Arnold 2007), and sodefrin is a known reproductive pheromone in newts (Kikuyama, Toyoda, Ohmiya, Matsuda, Tanaka and Hayashi 1995 Kikuyama and Toyoda 1999). Second, a separate study showed that the cDNA library of proteins expressed in male D. ocoee mental glands contained a high proportion (25%) of... 

FI CURE 3.4 Glands in male frogs, (a) Mental gland of Kassinasenegalensis ... 

Short-range transfer of chemical factors may require body contact. Male Appalachian woodland salamanders press or slap their mental gland on the nares of the female (Arnold, 1966). The pygmy salamander even pierces the skin of the female s head with his modified teeth and vaccinates it with the secretion from his mental gland. 

Courtship pheromones are not necessarily species specific. Pairs of the woodland salamander, Plethodon shermani, courted for an equally long time (about 35 to 50 minutes) whether male pheromone from the mental gland of conspecifics or the allopatric species P. montanus or P. yonahlosscc was present, even though the composition of the proteinacous pheromones (plethodontid receptivity factor of these three species differ considerably (Rollmann et al, 2003). 

Another salamander courtship pheromone, identified in 1999, is a protein. During courtship, males of the Appalachian woodland salamander, Plethodon jordani, (Plethodontidae), a terrestrial species, actively deliver a pheromone to the female. In the mating season, males develop a mental gland, located... 

Male courtship pheromones can increase the receptivity of the female. In the plethodontid salamander Desmognathus ochrophaeus the male courts the female hy scraping her dorsum with his specialized premaxillary teeth and swahs the same area with secretion from the mental gland on his chin, amounting to an injection. In an experiment, filter paper with an extract from the excised mental glands of males was placed on the dorsum of females. These treated females mated 28% (59 minutes) sooner than controls (Houck and Reagan, 1990). 

The terrestrial salamander Plethodonjordani applies courtship pheromone to the female hy ruhhing or slapping his mental gland directly on the female s nares. This way the pheromone can stimulate the vomeronasal organ and accessory olfactory system. Experimental application of mental gland extracts to the nares of females accelerated the time until active courtship started (Houck etal., 1998). 

In most plethodontid salamanders, courtship is terrestrial and the male does not clasp the female or restrain her in any way. The male, nevertheless, makes much physical contact with the female, crawling over and under her, or rubbing her snout with his head. In particular, the male frequently delivers secretions from specialized cephalic glands directly to the female. In Plethodon jordani, the male has an enlarged mental (submandibular) gland that he slaps over the female s nares (Arnold, 1977, Fig. 8). In a related species, yonahlossee, the male also slaps his mental gland over the female s snout (Arnold, 1972). Neither P. jordani nor P. yonahlossee males use their teeth to aid in secretion delivery. 

The only other experimental attempt to evaluate the effects of male courtship pheromones in salamanders is work conducted with the terrestrial plethodontid, Desmognathus ochrophaeus (Houck and Arnold, in prep.). Courtship interactions for this species (described above) involve pheromone delivery from the male s mental gland, and the probable use of premaxillary teeth to introduce this pheromone into the female s circulatory system. In their experiment, Houck and Arnold had three experimental male groups (1) Each male had its mental gland surgically removed, (2) Each... 

---