Non-Breeding Eusocial Mole-Rats Produce Viable Sperm—Spermiogram and Functional Testicular Morphology of Fukomys anselli

Ansell’s mole-rats (Fukomys anselli) are subterranean rodents living in families composed of about 20 members with a single breeding pair and their non-breeding offspring. Most of them remain with their parents for their lifetime and help to maintain and defend the natal burrow system, forage, and care for younger siblings. Since incest avoidance is based on individual recognition (and not on social suppression) we expect that non-breeders produce viable sperm spontaneously. We compared the sperm of breeding and non-breeding males, obtained by electroejaculation and found no significant differences in sperm parameters between both groups. Here, we used electroejaculation to obtain semen for the first time in a subterranean mammal. Spermiogram analysis revealed no significant differences in sperm parameters between breeders and non-breeders. We found significantly larger testes (measured on autopsies and on living animals per ultrasonography) of breeders compared to non-breeders (with body mass having a significant effect). There were no marked histological differences between breeding and non-breeding males, and the relative area occupied by Leydig cells and seminiferous tubules on histological sections, respectively, was not significantly different between both groups. The seminiferous epithelium and to a lesser degree the interstitial testicular tissue are characterized by lesions (vacuolar degenerations), however, this feature does not hinder fertilization even in advanced stages of life. The continuous production of viable sperm also in sexually abstinent non-breeders might be best understood in light of the mating and social system of Fukomys anselli, and the potential to found a new family following an unpredictable and rare encounter with an unfamiliar female (“provoked or induced dispersal”). Apparently, the non-breeders do not reproduce because they do not copulate but not because they would be physiologically infertile. The significantly increased testes volume of breeding males (compared to non-breeders) is in agreement with previously found higher testosterone levels of breeders

Higher gene expression stability during aging in long-lived giant mole-rats than in short-lived rats

Many aging-associated physiological changes are known to occur in short- and long-lived species with different trajectories. Emerging evidence suggests that numerous life history trait differences between species are based on interspecies variations in gene expression. Little information is available, however, about differences in transcriptome changes during aging between mammals with diverging lifespans. For this reason, we studied the transcriptomes of five tissue types and two age cohorts of two similarly sized rodent species with very different lifespans: laboratory rats (Rattus norvegicus) and giant mole-rats (Fukomys mechowii), with maximum lifespans of 3.8 and more than 20 years, respectively. Our findings show that giant mole-rats exhibit higher gene expression stability during aging than rats. Although well-known aging signatures were detected in all tissue types of rats, they were found in only one tissue type of giant mole-rats. Furthermore, many differentially expressed genes that were found in both species were regulated in opposite directions during aging. This suggests that expression changes which cause aging in short-lived species are counteracted in long-lived species. Taken together, we conclude that expression stability in giant mole rats (and potentially in African mole-rats in general) may be one key factor for their long and healthy life

Low sulfide levels and a high degree of cystathionine β-synthase (CBS) activation by S-adenosylmethionine (SAM) in the long-lived naked mole-rat

Hydrogen sulfide (H2S) is a gaseous signalling molecule involved in many physiological and pathological processes. There is increasing evidence that H2S is implicated in aging and lifespan control in the diet-induced longevity models. However, blood sulfide concentration of naturally long-lived species is not known. Here we measured blood sulfide in the long-lived naked mole-rat and five other mammalian species considerably differing in lifespan and found a negative correlation between blood sulfide and maximum longevity residual. In addition, we show that the naked mole-rat cystathionine β-synthase (CBS), an enzyme whose activity in the liver significantly contributes to systemic sulfide levels, has lower activity in the liver and is activated to a higher degree by S-adenosylmethionine compared to other species. These results add complexity to the understanding of the role of H2S in aging and call for detailed research on naked mole-rat transsulfuration

Unusual Ratio between Free Thyroxine and Free Triiodothyronine in a Long-Lived Mole-Rat Species with Bimodal Ageing

<div><p>Ansell's mole-rats (<i>Fukomys anselli</i>) are subterranean, long-lived rodents, which live in eusocial families, where the maximum lifespan of breeders is twice as long as that of non-breeders. Their metabolic rate is significantly lower than expected based on allometry, and their retinae show a high density of S-cone opsins. Both features may indicate naturally low thyroid hormone levels. In the present study, we sequenced several major components of the thyroid hormone pathways and analyzed free and total thyroxine and triiodothyronine in serum samples of breeding and non-breeding <i>F. anselli</i> to examine whether <i>a</i>) their thyroid hormone system shows any peculiarities on the genetic level, <i>b</i>) these animals have lower hormone levels compared to euthyroid rodents (rats and guinea pigs), and <i>c</i>) reproductive status, lifespan and free hormone levels are correlated. Genetic analyses confirmed that Ansell's mole-rats have a conserved thyroid hormone system as known from other mammalian species. Interspecific comparisons revealed that free thyroxine levels of <i>F. anselli</i> were about ten times lower than of guinea pigs and rats, whereas the free triiodothyronine levels, the main biologically active form, did not differ significantly amongst species. The resulting fT4:fT3 ratio is unusual for a mammal and potentially represents a case of natural hypothyroxinemia. Comparisons with total thyroxine levels suggest that mole-rats seem to possess two distinct mechanisms that work hand in hand to downregulate fT4 levels reliably. We could not find any correlation between free hormone levels and reproductive status, gender or weight. Free thyroxine may slightly increase with age, based on sub-significant evidence. Hence, thyroid hormones do not seem to explain the different ageing rates of breeders and non-breeders. Further research is required to investigate the regulatory mechanisms responsible for the unusual proportion of free thyroxine and free triiodothyronine.</p></div

fT4:tT4 and fT3:tT3 ratios in Ansell's mole-rats (n = 12), rats (n = 7) and guinea pigs (n = 4).

<p>One-way ANOVA, fT4:tT4: <i>F = </i>9.60, <i>p</i> = 0.001; fT3:tT3: <i>F = </i>7.724, <i>p = </i>0.004. Significant differences in the Bonferroni post hoc comparisons are indicated by asterisks coupled with the comparison species referred to (mr  =  mole-rat, r =  rat, gp  =  guinea pig) in parenthesis. See “Results” section for statistical details.</p

Free T4 and free T3 levels of Ansell's mole-rats (n = 24), rats (n = 4) and guinea pigs (n = 4).

<p>Mean ± SD; all data expressed in pg/ml. One-way ANOVA, fT4: <i>F</i> = 206.38, <i>p</i><0.0001; fT3: <i>F</i> = 0.85, <i>p</i> = 0.44. Significant differences in the Bonferroni post hoc comparisons are indicated by asterisks coupled with the comparison species referred to (mr  =  mole-rat, r =  rat, gp  =  guinea pig) in parenthesis. See “Results” section for statistical details and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113698#pone-0113698-t003" target="_blank">Table 3</a> for TH ratios obtained from these data. Mole-rat fT4 data refer to scenario 1; applying the second scenario (not depicted here) created essentially the same result because mean fT4 values of mole-rats were slightly lower.</p

Intraspecific fT4 (both scenarios) and fT3 differences in Ansell's mole-rats.

<p>GLM main effects for all four factors alone and a GLM two factor model with reproductive status × age as independent variable. The <i>p</i>-values and the correlation coefficients <i>F</i> are shown.</p><p>* =  significant (<i>p</i><0.05).</p><p>Intraspecific fT4 (both scenarios) and fT3 differences in Ansell's mole-rats.</p

Total T4 and total T3 levels of Ansell's mole-rats (n = 12), rats (n = 7) and guinea pigs (n = 4).

<p>Mean ± SD; all data expressed in ng/ml. One-way ANOVA, tT4: <i>F = </i>19.51, p<0.0001; fT3: <i>F = </i>11.13, <i>p</i> = 0.001. Significant differences in the Bonferroni post hoc comparisons are indicated by asterisks coupled with the comparison species referred to (mr  =  mole-rat, r =  rat, gp  =  guinea pig) in parenthesis. See “Results” section for statistical details and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113698#pone-0113698-t003" target="_blank">Table 3</a> for TH ratios obtained from these data.</p

Normal morphology of Ansell’s mole-rat sperm.

<p>The samples were obtained by electroejaculation (light microscopy, magnification 600x, oil immersion, eosin & nigrosin staining).</p

Main parameters (mean, SD given in parenthesis) of male reproductive characteristics measured in Ansell’s mole-rats.

<p>Volumes estimated for the right and left testis of each male were averaged to obtain mean absolute and relative testes volumes, respectively. *The minimum age at the time of testis measurement was estimated for two specimens since they were caught in the field in Lusaka, Zambia.</p