Using niche centrality within the scope of the nearly neutral theory of evolution to predict genetic diversity in a tropical conifer species-pair

Aim: Estimating genetic diversity is key for understanging biogeographic and evolutionary processes. However, gathering genetic information is not feasible for all taxa or populations, particularly in the tropical regions. Identifying proxies for inferring such values has thus become essential. Here, we built on the niche centrality hypothesis (NCH; or central-abundance hypothesis) and the nearly neutral theory of evolution (NNT) to identify some of such proxies using a montane tropical conifer species-pair as model. The NCH predicts more genetic diversity under optimal ecological conditions, which should also allow for more efficient purifying selection, according to the NNT.

Location: The Transmexican Volcanic Belt, central Mexico.

Taxa: A fir species-pair endemic to central Mexico, Abies flinckii and A. religiosa.

Methods: We estimated patterns of genetic diversity from nuclear SSRs (A, HE), and gene-coding sequences (πS, πN), together with the efficacy of purifying selection, measured as πN/πS. After testing for niche overlap, we used several geographic and ecological proxies (i.e. longitude, latitude, elevation, estimated area, and distance to the niche centroid in the present and in the LGM) to predict genetic diversity and πN/πS using general linear models.

Results: Populations at the west of the Trans Mexican Volcanic Belt (TVB) had lower genetic diversity than populations in the east of this mountain chain. Both species had significant niche overlap. The principal predictors for neutral genetic diversity (HE, A and πS) were longitude and latitude, followed by the current distance to the niche centroid; the efficiency of purifying selection was mostly accounted for by the current distance to the niche centroid (which was also correlated to elevation). No correlation was observed between genetic diversity or πN/πS and current population area.

Main conclusions: Historical and ecological factors have to be taken into account for explaining the amounts of genetic diversity in mountain tropical species. Following the NTT, populations closer to the niche centroid are more efficient at eliminating slightly deleterious mutations than marginal stands, independently of their size or geographical location (longitude). Expanding the central-abundance theory within the scope of the NTT might help reconciling conflicting views concerning the extent of its empirical support.

Methods
We sampled needles for natural populations of two species Abies flinckii and A. religiosa in central Mexico. We sequenced 11 gene-coding regions with Sanger sequencing and four nucelar SSRs. The table 1 shows details (n= sample size).

Population
Species
n =Sequencing coding regions
n= Nuclear SSRs
Longitude
Latitude
Elevation

1 (AF14)
A. flinckii
4
20
-100.75
19.583
2340

2 (AF16)
A. flinckii
7
20
-101.35
19.333
2250

3 (AF17)
A. flinckii
7
18
-102.95
18.767
2500

4 (AF18)
A. flinckii
5
22
-104.716
20.2
2100

5(AF19)
A. flinckii
7
20
-105.000
20.35
2490

6 (AF20)
A. flinckii
7
20
-103.933
19.45
2500

8 (AR12)
A. religiosa
7
21
-99.80
19.183
3240

9(AR13)
A. religiosa
7
20
-100.18
19.433
2800

10 (AR15)
A. religiosa
7
21
-100.817
19.667
2880

11 (AR21)
A. religiosa
7
19
-103.95
19.45
2500

12 (AR22)
A. religiosa
7
18
-103.583
19.583
3330

13 (AR45)
A. religiosa
7
22
-97.35
18.967
3060

14 (AR48)
A. religiosa
7
21
-97.15
19.517
3510

15 (AR49)
A. religiosa
7
19
-98.083
19.683
2760

16 (AR53)
A. religiosa
7
19
-98.7
20.15
2940

17 (AR54)
A. religiosa
7
20
-98.683
19.083
3330

18 (AR55)
A. religiosa
7
20
-102.317
19.383
3030

19
A. religiosa
7
20
-99.267
19.223
3369

20
A. religiosa

19
-98.055
19.253
3358

21
A. religiosa

20
-103.642
19.521
2928